A Manual of Elementary Geology

- Calcareous - Gypsum - Forms of stratification - Original hor

ain fossils. We may first study them with reference to their mineral composition, external appearance, position, mode of origin, organic contents, and other characters which belong to

ials of which such strata are composed. These may be said to belong principally to three divisions, the arenaceous, the argillaceous, and the calcareous, which are formed respectively of sand, clay, and carbonate of lime. Of these, the arenaceous, or sandy masses, ar

as quartz and common flint. Quartz is silex in its purest form; flint usually contains some admixture of alumine and oxide of iron. The siliceous grains in sand are usually rounded, as if by the action of running w

by ordinary pressure. In nature there is every intermediate gradation, from perfectly loose sand, to the hardest sandstone. In micaceous sandstones mica is very abundant; and

to be called pebbles, it becomes a conglomerate, or pudding-stone, which may consist of pieces of one o

r by the potter's lathe, is called a clay; and such clays vary greatly in their composition, and are, in general, nothing more than mud derived from the decomposition or wearing down of various rocks. The purest clay found in nature is porcelain clay, or kaolin, which results from the decom

breathed upon, which is a test of the presence of alumine, although it does not belong to p

ine these calcareous substances, that is to say, to expose them to heat of sufficient intensity to drive off the carbonic acid, and other volatile matter, without vitrifying or melting the lime itself. White chalk is often pure carbonate of lime; and this ro

r. These last might be called "calcareous sandstones;" but that term is more properly applied to a rock in which the

ns, resembling the roe of a fish, each of which has usually a small fragment of sand

siliferous; but statuary marble, which is also called saccharine limestone, as having a texture resemb

carbonate of lime and flint, and is harder i

th them to form new compounds, thereby becoming a sulphate, nitrate, or muriate of lime. The carbonic acid, when thus liberated from its union with the lime, escapes in a gaseous form, and froths up or effervesces as it makes its way in small bubbles through the drop of liquid. This efferve

of Alum Bay in the Isle of Wight, or sandstone so pure as the grit of Fontainebleau, used for pavement in France. More commonly we find sand and clay, or clay and marl, intermixed in the same mass. When the sand and clay are each in considerable quantity, the mixture is called loam. If there is much calcareous matter in clay it is called marl; but this term has unfortunately been used so vaguely, as often to be ve

ing a calcareous shale. It is very abundant in some countries, as in the

is composed of carbonate of lime and carbonate of magnesia; the proportion of the latter amounting in some cases to nearly one half. It effervesces much more slowly and feebly with acids than common limestone. In England this rock is generally of a yellowish

not contain carbonic acid gas, or fixed air, the lime being already combined with sulphuric acid, for which it has a stronger affinity than for any other. Anhydrous gypsum is a rare variety, into which water does not enter as a component part. Gypseous marl is a mixture of gypsum and marl. Alabaster is a gra

arser grain, some white, others of a dark colour, and below these, layers of shale and sandstone or beds of shale, divisible into leaf-like lamin?, and containing beautiful impressions of plants. Then again we meet with beds of pure and impure coal, alternating wit

eatise on the modern changes of the earth's surface.[14-A] It is there seen that rivers flowing into lakes and seas are charged with sediment, varying in quantity, composition, colour, and grain according to the seasons; the waters are sometimes flooded and rapid, at other periods low and feeble; different tributaries, also, draining peculiar countries and soils, an

r, it is found to consist of brown laminated clay, divided by thin seams of mica. The separation of the mica in this case, or in that of micaceous sandstones, may be thus understood. If we take a handful of quartzose sand, mixed with mica, and throw it into a clear running stream, we see the materials immediately sorted by the water, the grains of quartz falling almost directly to the bottom, while the plates of mica take a much longer time to reach the bottom, and are carried farther down the stream. At the first instant the water is turbid, but immediately after the flat surfaces of th

layers. The reason of this arrangement can by no means be attributed to an original evenness or horizontality in the bed of the sea; for it is ascertained that in those places where no matter has been recently deposited, the bottom of the ocean is often as uneven as that of the dry land, having in like manner its hills

he bottom, and causes them to settle in hollows or depressions, where they are less exposed to the force of a current than when they are resting on elevated points. T

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ommodated themselves in a great degree to the shape of the ground A B, yet each bed is thickest at the bottom. At first a great many particles would be carried by their own gravity down the steep sides of A and B, and others would afterwards be blown by the wind as they fell off the ridges, and would settle in the hollow, which would thus become more and more effaced as the strata accumulated from c to e. This levelling operation may perhaps be rendered more clear to the student by supposing a number of parallel trenches to be dug in a plain of moving sand, lik

y cause irregularities. We may sometimes follow a bed of limestone, shale, or sandstone, for a distance of many hundred yards continuously; but we generally find at length that each individual stratum thins out, and allows the beds

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of sandstone, grit

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ar Biggleswade, Bedfordshire. Hei

cannot possibly be accounted for by any re-arrangement of the particles acquired during the consolidation of the rock. In what manner then can such irregularities be due to original deposition? We must suppose that at the bottom of the sea, as well as in the beds of rivers, the motions of waves, currents, and eddies often cause mud, sand, and gravel to be thrown down in heaps on particular spots, instead of being spread out uniformly over a wide area. Sometimes, when banks are thus formed, currents may cut passages through them, just as a river forms its bed. Suppose the bank A (fig. 4.) to be thus formed with a steep sloping side, and the water being in a tranquil state, the layer of sediment No. 1. is thrown down upon it, conforming nearly to its surface. Afterwards the other layers, 2, 3, 4, may be deposited in succession, so

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g.

g.

en Mismer a

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o to the sea by the val

sandstone. (Gree

ds of grave

nd of St. Madeleine

ns, as in the annexed fig. 7., where a vast succession of slanting beds of gravel and sand may be traced from the sea to Monte Calvo, a distance of no less than 9 miles in a straight line. The dip of these beds is remarkably uniform, being always southward or towards the Mediterranean, at an angle of about 25°. They are exposed to view in nearly vertical precipices, varying from 200 to 600 feet in height, which bound the valley through which the river Magnan flows. Although in a general view, the strata appear to be parallel and uniform, they are nevertheless found, when examined closely,

m of great shoals terminating in a steep talus; such being the original mode of accumulation of all coarse materials conveyed into deep water, especially where they are composed in great part of pebbles, which cannot be transported to indefinite distances by currents of moderate velocity. By inattention to facts and inferences of this kind, a very exaggerated estimate has sometimes been made of the supposed depth of the ancient ocean. There can be no doubt, for example, that the strata a, fig. 7., or those nearest to Monte Calvo, are older than th

n the deposit of the Magnan was formed, the shape and outline of the alpine declivities and the shore greatly resembled what we now behold at many points in the neighbourhood. That the beds, a, b, c, d,

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ked (new red) sand

this ripple over an area of several yards square, I saw them perfectly restored in about ten minutes, the general direction of the ridges being always at right angles to that of the wind. The restoration began by the appearance here and there of small detached heaps of sand, which soon lengthened and joined together, so as to form long sinuous ridges with intervening furrows. Each ridge had one side slightly inclined, and the other steep; the lee-side being always steep, as b, c,-d, e; the windward-side a gentle slope, as a, b,-c, d, fig. 9. When a gust of wind blew with sufficient force to drive along a cloud of sand, all the ridges were seen to be in motion at once, each encroaching on the furrow before it, and, in the course of a few minutes, filling the place which the furrows had occupied. The mode of advance was by the continual drifting of grains of sand up the slopes a b and c d, many of which grains, when they arrived at b and d, fell over the scarps b c and d e, and

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ends to a very slight depth. To this rule, however, there are some exceptions, and recent ripple marks have been observed at the depth of 60 or 70 fe