Order VI.-Iron Sand and Chlorite Chalk. Order I-London, Paris, and Plastic Clay, sins. Sand, lignite, and salt water shells. Order Il-First Tertiary Blue London Clay, Limestone. Chloritic Limestone. Order III-First Local brack- Marls, Order IV-Second Tertiary Marls, Limestone. Class VI.-VOLCANIC PRODUCTS. Burh-stones of Paris basin, and Isle of Wight. Order I.-Basaltic Rocks. Basalt, Order II.-Lava. Porphyry. Lava, It will be observed in this classification, that the same formation or kind of rock sometimes occurs more than once, or is arranged under several different classes or orders. Thus limestone is sometimes primitive, at others, transition, secondary, or tertiary; and clay-slate and sandstone are sometimes associated with one formation, and sometimes with another. When, therefore, a rock, under the same name, is supposed, by its associations, to have been formed at different periods, it is classed severally with those of its own age. Thus limestone is of all ages, and consequently belongs to all the classes, except the volcanic. The same is more or less the case with sandstone and clayslate, and several others. COMPARATIVE AGES OF ROCKS. We have already noticed, under the descriptions of the different formations, their relative ages, but a recapitulation is required in order to bring this subject distinctly before the reader. It requires no arguments to show that the lowest formations must be the oldest, since these must have been deposited before those which lie above, or upon them. It is true that a mountain of granite, when shaken, or uplifted by an earthquake, may fall and spread its ruins on the plain below, but such an occurrence would readily be detected, since the situation of its fragments would show that this was not an original and undisturbed formation. Granite and its associates, besides being placed lowest in the order of position, are, as we have already seen, entirely destitute of organic remains. It ought, however, to be noticed that Dr. Macculloch, in a single instance, in one of the Hebrides, observed gneiss overlaying a bed of limestone, which contained bivalve shells. But the ex treme contortions of the gneiss, on that island, are sufficient to show that a bed, really superior in its general position, may appear to be inferior at some particular points Fig. 21. Thus let a a a, fig. 21, be the contorted substratum of gneiss, and b, c, d, e, a superior and incumbent bed of organic limestone, following its flexures. Now it is clear, that if these beds be visi ble only at the point d, the limestone will appear to be below the gneiss, though the error would readily be corrected by an examination at any other point. Such apparent exceptions do not, however, affect the general fact, for nothing in geology is more clearly established, than that granite, and its associates, lie below all other rocks, and hence must be older than any of their super-strata. The transition rocks come next to granite, with respect to position, and, consequently, with respect to antiquity. In these, organic remains begin to occur, as plants and shells. Next to these are the lower, and then the upper secondary rocks. In these are found fossil relics in great quantities, as shells, fish, and some of the amphibious tribes. Above the secondary come the tertiary strata, and in these formations, are found the bones of quadrupeds of extinct species. Volcanic products are both of ancient and modern date. Diluvial deposites are supposed to be of no greater antiquity than the Noachian deluge, having been formed entirely by that catastrophe. In these, the remains of huge quadrupeds, as the elephant, mastodon, and rhinoceros, are found. Alluvial products are the most recent in the order of strata; being, like volcanic products, constantly forming. STRATA AND STRATIFICATION. Most secondary, and several primitive rocks, are composed of layers, or portions, resting one above another, with seams between them. These portions, or layers, are called strata, and formations of this kind are called stratified. In general, such rocks are fissile, and may be divided into flat tables, or layers, in the direction of their strata. These rocks have apparently been formed by gradual depositions from water, accumulated one upon the other. Unstratified rocks show no signs of such gradual accumulation; they present no lines of stratification, nor are they fissile in one direction more than in another; such are granite, greenstone, and basalt. Fig. 22. Strata are said to be horizontal, when they coincide with the direction of the horizon, or have little or no inclination, as represented by fig. 22. It is very rare, however, that such strata are found, except among the most recent deposites, the secondary or tertiary strata, in nearly every instance being more or less inclined. Dip. The inclination of strata from a horizontal position is called their dip, the amount of the dip being the quantity of the angle, which the line of inclination makes with that of the horizon. This is represented by fig. 23. If the angle made by the meeting of Fig. 23. the lines of the strata, b b, and the a horizontal line a, be equal to 45° towards the east, then the strata are said to dip 45° in that direction. Outcrop. When strata protrude above the surface, or are uncovered, as on the side of a hill, so as to be seen, they are said to crop out. The uncovered ends of the strata commonly rise above each other, like stairs, or, as Mr. Bakewell has it, like a number of slices of bread and butter, laid inclined on a plate. In fig. 23, the outcrop of strata is represented at bb. Outcrop is a matter of much importance to geologists and practical miners, since the upper, as well as the under, strata may be observed at these points; and thus, without excavations or borings, not only the dip can be ascertained, but also the different kinds of rock with which a country is underlaid. |