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.

a

Fig. 21

a

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, fol lowing 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 com

posed 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 depositious 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 and 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.

Fig. 23.

a

If the angle made by the meeting of the lines of the strata,

bb, and the 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 b b. 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.

Outlier. Strata are said to form outliers, when they constitute a portion of country detached from the main

Fig. 24.

mass of the same bed of which they evidently once formed a part. Thus the bed b, fig. 24, on the top of the hill, is an outlier of the main stratum a, the in

tervening valley being scooped out, either by the general deluge, or some other means. The kind, and thickness, as well as the range of the intercepted strata, are sufficient to prove that they were once continuous.

Escarpment. Strata are said to terminate in an escarpment, when they end abruptly, as at a b, fig. 24.

Mural precipice. Mural signifies wall-like, and rocks are said to form such precipices, when they present naked, and nearly perpendicular faces.

Conformable position. Strata
Fig. 25.

are said to be conformable, when their general planes are parallel, whatever their dip may be. Fig. 25, a a, reprea sents conformable strata, as shown by their parallel planes.

Unconformable Strata. When a series of upper strata, rest on a lower formation, without any conformity to the position of the latter, the upper series is called unconformable, as represented at b b, fig. 25.

Fault. This is such a dislocation of the strata, that not only their continuity is destroyed, but the series of beds on one or both sides of the fractures, are forced out of their original positions, so that it often happens in mining for coal, the workmen suddenly come to the apparent termina tion of the vein by a wall of rock.

Dyke. This is a wall of rock interposed between the two sides, or ends of a dislocation, and in consequence of which, the continuity of the beds or strata are interrupted.

If we suppose that the dyke was once fused matter, forced up from beneath, and that on one of its sides the strata were elevated, or on the other depressed by a subterranean convulsion, it would account for the phenomena both of the fault and the dyke.

fore the workmen search on the opposite side of the dyke, for the coal vein, they find instead of coal, perhaps sandstone or clay, and thus for a time, the work of the mine is entirely suspended, the coal being lost. In attempting to regain the vein, the first question to be determined is, whether it has been thrown up, or cast down on the other side of the dyke; and this in general, is readily decided by the position of the dyke, or its inclination with respect to the fault. For experience has shown, 'that if the dyke makes an acute angle with the upper surface of the coal vein, the strata are elevated on that side, while if the angle is obtuse, they are thrown down, as represented by fig. 26.

In some coal fields, the strata are raised or depressed on one side of the dyke, to the extent of four or five hundred feet.

Dykes which intercept coal strata are most frequently composed of basalt, but sometimes of indurated clay. They are, in thickness, from a few inches to fifty or sixty feet, and in a few instances are three hundred feet thick. Dykes are seldom noticed except in mining districts, where they excite much interest in consequence of the disturbances they occasion to coal veins. Their extent therefore is generally quite uncertain, though in some instances they are known to traverse large sections of country.

Dykes being generally impervious to water, they obstruct its passage along the porous strata, and occasion it to rise towards the surface; hence it frequently happens that numerous springs make their appearance along the course of a dyke, which is entirely under ground, and by which alone its existence is indicated.

Slaty Structure. Professor Sedgwick has made some curious and important observations on the difference between the planes of stratification and those of cleavage, as applicable particularly to the roofing-slate of Wales.

In mica-slate, the cleavage is in the direction of the strata of deposition, whether the layers are curved or not, and the same is the case with common clay-slate, and in depositions of clay which are separable in layers. In beds of roof-slate the case is quite different, the cleavage being not in the direction of the strata, but in general, obliquely across them. The strata are seldom or never either horizontal, or straight, but contorted, bent, or waved, and are often far from being parallel with each other.

Fig. 27.

Professor Sedgwick gives the diagram fig. 27 in illustration of this subject, and remarks "that the contortions of slate rocks are phenomena quite different from cleavage, and the curves presented by such formations are the true lines of disturbed strata." The contorted lines running lengthwise the diagram are the true strata, while those crossing these in nearly a vertical direction, and preserving almost a geometrical parallelism are the lines. of cleavage. A region of more than thirty miles in length, and eight or ten in breadth, exhibits this structure on a magnificent scale. Many of the contorted strata are of a coarse mechanical structure; but subordinate to them are fine crystalline, chloritic slates. But the coarser beds and the finer, the twisted and the straight, have all been subjected to one change. Crystalline forces have re-arranged whole mountain masses of them, producing a beautiful crystalline cleavage, passing alike through all the strata; and through all this region whatever may be the contortions, the planes of cleavage pass on, generally without deviation, running in parallel lines from one end to the other.

"Without considering the crystalline flakes along the planes of cleavage, which prove that crystalline action