That is not all, however. In the same brick-earths and gravels28, tools (flint implements29), fashioned by the hands of man, are also frequently discovered, and in one place at Crayford, the spot whereon flint implements were manufactured has been ’lighted upon. Each flake30 chipped off has been collected and pieced together, and the shape of the original flint has thus been determined31. Clearly, from this evidence, the earth from which millions of bricks have been made has formed since prim32?val man (and with him the animals alluded33 to)4 inhabited the valleys of the Thames and its tributaries34. It is interesting, too, to reflect on the circumstance that the materials upon which many of these facts of great philosophical35 significance are based, have been collected through the instrumentality of the workmen. Pal36?ontologists are proud to acknowledge that; their debt of gratitude37 to the intelligent and persevering38 men can never be fully39 repaid.
Pursuing the matter still further, we discover a quantity of shells, blanched40 and very frail41—they seem to be deprived of much of their original substance, so to speak; their entombment in the brick-earth has taken all the natural colour out of them. Studying these, we soon ascertain that they belong to land snails42 and mollusca which inhabit fresh water. Living representatives of the same species are, with few exceptions, found in Kent and Essex.
Putting all this evidence together, we come to the conclusion that the brick-earths alluded to accumulated in the channel of a river; they are found above the present level of the Thames, for the simple reason that they have been elevated into that position partly by earth movements and partly by the channel of the river being cut deeper by natural causes, of which abundant proof will be adduced. The snails were washed down from the land by freshets, or caught by the river in flood; the elephant, rhinoceros, hippopotamus, and musk sheep were overcome, perhaps, by floods, drowned, and subsequently covered up by the mud of the swollen43 current. We can imagine that the savage44 hunter, in his canoe, attacking the animals swimming in the river, loses his tomahawk, or his frail bark may be upset, and he is striving to gain the shore for dear life. Or, it may be winter time; the river is frozen over, and he is cutting5 a hole in the ice with his flint chisel45 wherein to fish; his hands are benumbed, and he loses his grasp of the tool; it falls into the water, to be discovered in the brick-earth by one of our intelligent friends. Truly, the revelations of the brickyard enable us to construct a picture of one of the most interesting phases of the past history of the Earth.
We have given an outline of the evidence upon which certain brick-earths in the Thames valley are proved to be of fresh-water origin—to have accumulated in quiet reaches of the river, and at other convenient spots along its course—but we have used that as an illustration only; phenomena46 of precisely the same character are manifested in nearly all river valleys in this country, especially those in which the bottom of the valley has only a slight gradient down to the sea.
The brickmaker may ask: What is the practical bearing of these observations? What difference does it make to us whether the earths we use are of fresh-water, lacustrine, or marine48 origin? All the difference in the world, from the points of view of structure, composition and suitability of the earths, and especially of their distribution over the face of the country. How much easier it is to value an extensive brickmaking property when you feel perfectly49 certain as to whether the face of earth as shown in the pit will die out on being worked into for a few yards, or whether it will be persistent throughout the whole of the property to be valued. Better still, when your knowledge enables you to state definitely whether the quality of earth now being worked in a pit is likely to continue the same, or whether it will get better, or worse. The disposition50 of the earths, in some instances, is so clear that no brickmaker with an eye to business could fail to trace their6 extent over his property. But this is not often the case, for the earths being used are for the most part covered by a superficial mantle51, or overburden, which masks the true character of the beds beneath. A very slight acquaintance with the principles of geology overcomes these difficulties as a rule; and we are about to lay down the elements of these principles, so far as they apply to the immediate52 subject in hand. By seeing why it is the beds of brick-earth vary in structure and composition we shall be in a better position to make forecasts of their general behaviour.
In regard to fluviatile deposits, it goes without saying that every river flows along a general depression more or less pronounced, called a valley, and that this valley is bounded physiographically by a ridge53, except in the region of its entrance to the sea or lake, or, if a tributary54, of its joining a main stream. The watershed55 of a river and its tributaries includes and comprises what is technically56 termed the “river basin.” All valleys are, in the end, the result of denudation57 taking place in them. In other words, on the birth of a valley a very slight depression or other physical feature determined its general direction for the time being, but the little rivulet58 once being formed proceeded, through the medium of the “agents of denudation,” to carve out its channel more clearly, and eventually to eat into the rocks over which it flowed, until a large valley had been formed. The “agents of denudation” in river valleys may be summarised as rain, snow, ice, heat, and wind, and their general effect on rocks is called “weathering.” We need not stop to enquire into the precise methods adopted by these agents in accomplishing their work; it suffices at present to say that the rock destroyed or broken up is7 removed by the running water constituting the rivulet, stream, or river, as the case may be. Some of the material is chemically dissolved in the water, whilst another and larger proportion is taken away in suspension, or is said to be dealt with mechanically by the river. The agents of denudation do their work very slowly, as a rule, and yet no one who stands on London Bridge and contemplates59 the swollen stream laden60 with muddy sediment61 passing under it after a few days’ rain, could say that they are not doing their duty effectually. To give some idea of the quantity of sand, gravel, and mud removed from the land through the medium of rivers, we may remark that the Mississippi discharges into the Gulf62 of Mexico annually63 a mass of earthy matter equal to a prism 268 feet in height with a base area of one square mile. In regard to denudation by chemical means we may say that the Thames carries past Kingston 19 grains of mineral salts in every gallon of water, or a total of 1,502 tons every 24 hours, or 548,230 tons every year; this is not taking into account the muddy sediment, gravel, &c., annually sent down to the Nore, which must be infinitely64 greater in quantity.
Enough has now been said to show that stupendous quantities of mineral matter derived65 from the destruction of the land are sent down to the sea by natural agencies, and we may at once state that a very large proportion of this, which finds a resting-place in and about the mouths of the rivers and their backwaters, is material suitable for brickmaking at places where it is obtainable. Enormous quantities of muddy sediment, sand and gravel, however, never reach as far as the sea with great rivers. This material is arrested at sundry66 convenient spots, and, as a rule, forms excellent brick-earth.
See Fig. 1, which represents part of a river of slow8 current with three bends, A, B, C. The water is flowing in the direction indicated by the arrows; and it is part of the mechanics of such a river that in rounding a bend its velocity68 is greatest (and its eroding69 power also) at the outer portions of the curves approximately indicated by the arrow points. The water “wheels round” such portions of the curves, and “marks time” at the points x x x, and, indeed, its progress may be altogether arrested for a time at the latter places. Now the transporting power of a river is its velocity, and, naturally, the greater the velocity, the coarser will be the fragments or particles of rock carried along. It is interesting in this connection to quote the figures calculated by Mr. David Stephenson, giving the power of transport of different velocities70 of river currents:—
Ins. per
second. Mile per
hour.
3 = 0.170 will just begin to work on fine clay.
6 = 0.340 will lift fine sand.
8 = 0.4545 sand as coarse as linseed.
12 = 0.6819 will sweep along fine gravel.
36 = 2.045 will sweep along slippery angular stones of the size of an egg.
9 These figures2 have greater interest for us than in the connection at present used, as will be noticed hereafter. We have seen that in rounding the bends (Fig. 1) A, B, C, different portions of the stream possess different velocities. We know it is charged with sediment and stones all the time. The tendency, therefore, will be for the large stones and coarse detritus72 to go round the outer side of the bend, to bombard the banks near the points shown by the arrows, and to erode73 the channel deepest in those situations; whilst a goodly proportion of the fine muddy sediment will find its way to the quiet and shallow parts near x x x, and in course of time become deposited there, whilst the main course of the stream is eating its way and shifting its course as indicated by the dotted lines a a. This action proceeds, it may be, until the course of the river becomes straighter, as shown by the dotted lines b b, when the whole of the loop B D is abandoned, its former course there being evidenced by pools of water and irregular heaps of gravel, sand and mud. The reader will now see that the whole of the land marked x x x has been formed of sediment brought down by the river, and in the majority of cases such fine silt74 and sandy mud or clay is specially47 suitable for brickmaking—many of our largest brickmakers obtain their material from such a source. It should be observed that the valley, as shown between the lines v v, may be two or three miles in width, and it is often much more, so that the actual amount of land made by the river at x x x may be several thousands of acres in extent.
Now as to the practical application of the foregoing observations. In the first place, it will be seen that10 such deposits of brick-earth as are made in this manner cannot be very thick, their total thickness perhaps, resting on the bottom of the valley, not being more than 20 feet, and it is frequently much less. The next thing to be noticed is that they must be very variable in character, a bed changing perhaps every 100 feet or so horizontally, and more often every few feet. Individual beds must of necessity be very irregularly developed under the circumstances. The velocity of the stream being greater at certain seasons of the year than at others, we frequently find some such section as the following developed:—
Fig. 2.—Section of Fluviatile Brick-earth.
11
a = Mould and soil, of no use to the brickmaker.
b = Sandy clay, with a large proportion of sand; useful for moulding or incorporating with the “fat” clays below for brickmaking.
c = Gravel bed, lenticularly developed; suitable for mending roads, paths, &c.
d = Sandy clay; similar to b.
e = Thin bed of marl, with a fair proportion of lime.
f = Sands and small pebbles, irregularly stratified (false-bedded).
g = Laminated sandy clay.
h = Stiff clay; can be mixed with f and passed through the pug mill.
i = Sand; an irregular bed of very local occurrence.
j = Gravel bed, with much sand.
The above is typical of deposits accumulated in river valleys; it is different in character to deposits laid down in the sea (as will presently be described); the section exhibits very different classes of brick-earth also, and yields a totally different kind of brick to that obtainable from brick-earths of marine origin. The importance of the question of origin of a brick-earth, therefore, is just beginning to dawn upon us. Many rivers are noted75 as having throughout a long period of time wandered from one side of the valley to the other (by the process depicted76 in Fig. 1) several times, in which cases the brick-earth sections relating to them are liable to still greater variation. The reader would perhaps be very much astonished to find how much is known concerning peregrinations of that description in regard to particular localities, by competent authorities—usually field geologists77.
We come to another important point in regard to river deposits. The ceaseless flow of the river, and the abrading78 action of the large stones rolled along at the bottom of its channel, tend to cut the latter deeper12 and deeper, and we have excellent evidence that most English rivers once flowed at a greater elevation79 in their valleys than they now do. In consequence of this, the brickmaker may find his pit somewhat higher than the neighbouring river, which at an earlier stage of its existence made his brick-earths. To a certain extent, small earth movements, as previously80 explained, are also undoubtedly81 responsible for many of these brick-earths now being at a considerable elevation above the surface of the river. This phenomenon is illustrated82 in Fig. 3.
Fig. 3.—Section across a river valley, showing formation of terraces of gravel and brick-earth.
This type of disposition of fluviatile deposits is of common occurrence. We will assume that the valley is carved out of clay (shown by horizontal lines and dots). On both sides of it, and at the same relative heights, are two masses (marked 1 and 2) of brick-earths and gravels running along so as to form two distinct broad terraces. These beds were laid down when the river, in flood, though occupying only a small portion of the valley, was approximately of the height shown by the dotted lines a b. Denudation has been hard at work, however, since then, and only vestiges83 of these beds clinging to the sides of the valley, as shown, remain. At a later period, and coming on towards modern times, the13 broad expanse of beds (comparable in disposition with those depicted in Fig. 2) some miles in width, marked 3, were laid down, and we notice the river channel, as it now is, cutting its way through them. Thus it comes to pass that brickyards may be situated84 in terraces one above the other; and what is much more important, the brick-earths may vary very widely in quality along these horizons, those in 1 differing from 2 and both from 3. The brickyards may be quite close to each other, and to the unscientific eye the earths are of similar appearance, but they do not yield the same class of brick, and no one seems to trouble to enquire the reason why. These differences have resulted primarily from the materials having been derived from other collecting grounds, other watersheds85, than those comprised within the basin of the river as at present constituted. They are the inevitable86 accompaniment of the evolution of the river system, and throw light on successive phases of the past history of the stream and its tributaries. For us, as we have seen, they possess considerable practical value of the first importance in selecting the site for a brickyard.
Apart from differences of the character just described, serious alterations87 sometimes take place on these brick-earths being traced higher up the valley, and indeed an excellent brickmaking material may become absolutely worthless in that respect, for the reasons about to be explained. The reader will agree that neither stones nor sediment can travel up a valley, and he will understand that no sediment can be found in the valley earths other than that derived from the destruction of rocks within the watershed of the river system, to which the valleys belong, or did belong, at the time the earths were formed. We desire to put the case in a very simple light, so as to be clearly comprehended. Let us contemplate Fig. 4.
Fig. 4.—Map shewing river basin, with geological formations depicted.
14 Here we have represented a river basin, the limits (watershed) of which are indicated by a sinuous88 dotted line. Three geological formations are found therein; in the upper reaches of the main river is a series of clays marked A; a large tract89 in the middle, B, is sandstone; and the lower part, C, is occupied by limestone90. Seeing that nothing but clay crops out in the part A, it follows that the deposits of the river in that region must be principally of an argillaceous character, to the point a. On flowing over the sandstone B, the main stream, already charged with clay15 particles, will be mixed with sand; the proportion of sand increases as the first large tributary (b) to the east is encountered, and is considerably91 augmented92 as the still more important tributary (b) to the west enters it. The superficial deposits in the valleys of the area B will likewise be very sandy and perhaps gravelly at b b, but at c c1 the sands and gravels will be mixed with much clay. On passing over into the area C, much carbonate of lime is added, though the larger proportion denuded93 from the rocks is taken away, chemically, in solution. Nevertheless, nodules of “race” (lime concretions), limestone pebbles, and perhaps chert and flint gravel will come upon the scene at about the point marked e. At d the deposits would principally consist of gravel and impure94 marls. To sum up, the clays at a would no doubt be too stiff of themselves to make good bricks; similarly the beds at b b would be nothing but sand, though these might be made, with a little judicious95 treatment, into a species of fire-brick; at c we should find alternating loams and clays suitable for turning out fair bricks; at c1 the beds would be more variable in character and more locally developed; they would consist of thin beds of sand, clays, loams and gravels (principally sandstone fragments), which as a whole might be made serviceable, though difficult to deal with; nothing of much use to us would come from point d, nor bordering the tributary running over C; there would be too much lime present, though a trade might be started in basic bricks should there be any demand for them in the neighbourhood; this, however, would only pay under extremely favourable96 conditions. At e there may be a mixture of all the foregoing deposits, and providing the beds above were easily weathered and thick beds of loam were thus fairly well developed,16 good sites for brick-earth might be found. The point e might possess this advantage over the other sites mentioned, viz., that marls would no doubt be present, and thus no necessity should arise for grinding lime to be incorporated with the brick-earth; the only danger would be that lumps of limestone might be too numerous—especially if c were a hard limestone.
The general character of the deposits might be slightly modified by mineral matter brought up in springs and thrown down at convenient spots.
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1
ascertain
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| vt.发现，确定，查明，弄清 | |
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loams
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| v.（尤指适合植物生长的）壤土，沃土( loam的第三人称单数 ) | |
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loam
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| n.沃土 | |
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tinge
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| vt.(较淡)着色于，染色；使带有…气息；n.淡淡色彩，些微的气息 | |
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pebble
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| n.卵石，小圆石 | |
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gravel
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| n.砂跞；砂砾层；结石 | |
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scattered
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| adj.分散的，稀疏的；散步的；疏疏落落的 | |
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enquire
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| v.打听，询问；调查，查问 | |
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strata
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| n.地层（复数）；社会阶层 | |
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10
compensate
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| vt.补偿，赔偿；酬报 vi.弥补；补偿；抵消 | |
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persistent
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| adj.坚持不懈的，执意的；持续的 | |
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12
remains
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| n.剩余物，残留物；遗体，遗迹 | |
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13
jaw
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| n.颚，颌，说教，流言蜚语；v.喋喋不休，教训 | |
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14
skull
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| n.头骨；颅骨 | |
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exhumed
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| v.挖出，发掘出( exhume的过去式和过去分词 ) | |
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16
tusks
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| n.（象等动物的）长牙( tusk的名词复数 )；獠牙；尖形物；尖头 | |
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skulls
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| 颅骨( skull的名词复数 )； 脑袋； 脑子； 脑瓜 | |
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rhinoceros
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| n.犀牛 | |
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19
musk
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| n.麝香, 能发出麝香的各种各样的植物，香猫 | |
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20
grizzly
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| adj.略为灰色的，呈灰色的；n.灰色大熊 | |
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21
hippopotamus
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| n.河马 | |
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22
reindeer
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| n.驯鹿 | |
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23
precisely
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| adv.恰好，正好，精确地，细致地 | |
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24
latitudes
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| 纬度 | |
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25
denizen
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| n.居民，外籍居民 | |
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26
expatiating
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| v.详述，细说( expatiate的现在分词 ) | |
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contemplate
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| vt.盘算，计议；周密考虑；注视，凝视 | |
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gravels
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| 沙砾( gravel的名词复数 )； 砾石； 石子； 结石 | |
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29
implements
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| n.工具( implement的名词复数 )；家具；手段；[法律]履行（契约等）v.实现( implement的第三人称单数 )；执行；贯彻；使生效 | |
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30
flake
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| v.使成薄片；雪片般落下；n.薄片 | |
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31
determined
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| adj.坚定的；有决心的 | |
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32
prim
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| adj.拘泥形式的，一本正经的；n.循规蹈矩，整洁；adv.循规蹈矩地，整洁地 | |
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33
alluded
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| 提及，暗指( allude的过去式和过去分词 ) | |
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34
tributaries
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| n. 支流 | |
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35
philosophical
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| adj.哲学家的，哲学上的，达观的 | |
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36
pal
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| n.朋友，伙伴，同志；vi.结为友 | |
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37
gratitude
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| adj.感激，感谢 | |
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persevering
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| a.坚忍不拔的 | |
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fully
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| adv.完全地，全部地，彻底地；充分地 | |
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40
blanched
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| v.使变白( blanch的过去式 )；使（植物）不见阳光而变白；酸洗（金属）使有光泽；用沸水烫（杏仁等）以便去皮 | |
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41
frail
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| adj.身体虚弱的；易损坏的 | |
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snails
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| n.蜗牛；迟钝的人；蜗牛( snail的名词复数 ) | |
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swollen
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| adj.肿大的，水涨的；v.使变大，肿胀 | |
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savage
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| adj.野蛮的；凶恶的，残暴的；n.未开化的人 | |
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45
chisel
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| n.凿子；v.用凿子刻，雕，凿 | |
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46
phenomena
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| n.现象 | |
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47
specially
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| adv.特定地；特殊地；明确地 | |
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48
marine
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| adj.海的；海生的；航海的；海事的；n.水兵 | |
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49
perfectly
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| adv.完美地，无可非议地，彻底地 | |
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50
disposition
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| n.性情，性格；意向，倾向；排列，部署 | |
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51
mantle
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| n.斗篷，覆罩之物，罩子；v.罩住，覆盖，脸红 | |
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52
immediate
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| adj.立即的；直接的，最接近的；紧靠的 | |
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53
ridge
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| n.山脊；鼻梁；分水岭 | |
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54
tributary
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| n.支流；纳贡国；adj.附庸的；辅助的；支流的 | |
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55
watershed
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| n.转折点，分水岭，分界线 | |
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56
technically
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| adv.专门地，技术上地 | |
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57
denudation
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| n.剥下；裸露；滥伐；剥蚀 | |
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58
rivulet
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| n.小溪，小河 | |
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contemplates
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| 深思，细想，仔细考虑( contemplate的第三人称单数 )； 注视，凝视； 考虑接受（发生某事的可能性）； 深思熟虑，沉思，苦思冥想 | |
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laden
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| adj.装满了的；充满了的；负了重担的；苦恼的 | |
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sediment
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| n.沉淀，沉渣，沉积(物) | |
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gulf
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| n.海湾；深渊，鸿沟；分歧，隔阂 | |
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annually
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| adv.一年一次，每年 | |
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infinitely
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| adv.无限地，无穷地 | |
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derived
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| vi.起源；由来；衍生；导出v.得到( derive的过去式和过去分词 )；（从…中）得到获得；源于；（从…中）提取 | |
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sundry
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| adj.各式各样的，种种的 | |
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fig
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| n.无花果(树) | |
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velocity
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| n.速度，速率 | |
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eroding
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| 侵蚀，腐蚀( erode的现在分词 )； 逐渐毁坏，削弱，损害 | |
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velocities
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| n.速度( velocity的名词复数 )；高速，快速 | |
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pebbles
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| [复数]鹅卵石； 沙砾； 卵石，小圆石( pebble的名词复数 ) | |
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detritus
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| n.碎石 | |
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erode
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| v.侵蚀，腐蚀，使...减少、减弱或消失 | |
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silt
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| n.淤泥，淤沙，粉砂层，泥沙层；vt.使淤塞；vi.被淤塞 | |
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noted
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| adj.著名的，知名的 | |
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depicted
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| 描绘，描画( depict的过去式和过去分词 )； 描述 | |
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geologists
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| 地质学家，地质学者( geologist的名词复数 ) | |
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abrading
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| v.刮擦( abrade的现在分词 )；（在精神方面）折磨（人）；消磨（意志、精神等）；使精疲力尽 | |
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elevation
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| n.高度；海拔；高地；上升；提高 | |
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previously
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| adv.以前，先前（地） | |
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undoubtedly
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| adv.确实地，无疑地 | |
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illustrated
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| adj. 有插图的，列举的 动词illustrate的过去式和过去分词 | |
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vestiges
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| 残余部分( vestige的名词复数 )； 遗迹； 痕迹； 毫不 | |
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situated
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| adj.坐落在...的，处于某种境地的 | |
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watersheds
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| n.分水岭( watershed的名词复数 )；分水线；转折点；流域 | |
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inevitable
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| adj.不可避免的，必然发生的 | |
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alterations
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| n.改动( alteration的名词复数 )；更改；变化；改变 | |
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sinuous
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| adj.蜿蜒的，迂回的 | |
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tract
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| n.传单，小册子，大片(土地或森林) | |
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limestone
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| n.石灰石 | |
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considerably
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| adv.极大地；相当大地；在很大程度上 | |
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Augmented
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| adj.增音的 动词augment的过去式和过去分词形式 | |
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denuded
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| adj.[医]变光的，裸露的v.使赤裸( denude的过去式和过去分词 )；剥光覆盖物 | |
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impure
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| adj.不纯净的，不洁的；不道德的，下流的 | |
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judicious
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| adj.明智的，明断的，能作出明智决定的 | |
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favourable
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| adj.赞成的，称赞的，有利的，良好的，顺利的 | |
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