B. Aggregated a. Indeterminate b. Determinate I. Single 1. Granular 2. Slaty 3. Porphyritic 1. Granular slaty 2. Slaty granular 3. Granular porphyritic 4. Slaty porphyritic 5. Porphyritic and amygdaloidal SECT. II. Of the relative Situation of Rocks. The rocky masses, or rocks, hitherto observed, amount to about sixty. Of these rocks, variously placed over each other, the whole crust of the earth is composed, to the greatest depth that the industry of man has been able to penetrate; and with respect to each other, they occupy for the most part a determinate situation, which holds invariably in every part of the earth. Thus lime-stone is no where found under granite, but always above it. Were we to suppose every particular rock, or layer, which constitutes a part of the earth's surface to be extended round the whole earth, and to be wrapped round the central nucleus, like the coat of an onion, in that case every rock would be constantly found; one species would be always lowest or nearest the centre; another species would uniformly rest upon this first; a third upon the second, and so on. Now, though the rocks do not in reality extend round the earth in this uninterrupted manner; though, partly from the inequality of the nucleus on which they rest, partly from their own inequality of thickness in different places, and partly from other causes, the continuity is often interrupted; yet still we can trace enough of it to convince us that the rocks which constitute the earth's crust, considered in a great scale, are every where the same, and that they invariably occupy the same situation with respect to each other. Werner has therefore chosen this relative situation as the basis of his classification of rocks. He divides them into five VOL. I. Y classes. The first class consists of those rocks which, if we were to suppose each layer to be extended over the whole earth, would in that case lie lowest, or nearest the centre of all the rocks which we know to be covered by all the other rocks. The second class consists of those rocks which in that case would be immediately above the first class and cover them. The third class would cover the second in the same manner; the fourth the third; and the fifth would be uppermost of all, and constitute the immediate surface of the earth. The first class of rocks are covered by all the rest, but never themselves lie over any other. The others lie in order over each other. These grand classes of rocks he has denominated for. mations, and distinguished them by the following specific names: I. Primitive formations II. Transition formations III. Floetz, or horizontal formations IV. Alluvial formations V. Volcanic The primitive formations are of course the lowest of all, and the alluvial constitute the very surface of the earth; for the volcanic, as is obvious, are confined to particular points. Not that the primitive are always at a great depth under the surface, very often they are at the surface, or even constitute mountains. In such cases, the other classes of formations are wanting altogether. In like manner the transition, and other formations, may each in its turn occupy the surface, or constitute the mass of a mountain. In such cases, all the subsequent formations which ought to cover them are wanting in that particular spot. Each of these grand classes of formations consists of a greater or smaller number of rocks, which occupy a determinate position with respect to each other, and which, like the great formations themselves, may often be wanting in particular places. Let us take a view of the rocks which compose all these different formations. Class I. Primitive Formations. The rocks which constitute the primitive formations are very numerous. They have been divided therefore into seven sets, which constitute as many primitive formations, and are distinguished each by the name of that particular rock which constitutes the greatest proportion of the formation. These seven sets of primitive formations are the following: 1. Granite 2. Gneiss 3. Mica-slate 4. Clay-slate 55. Newest primitive porphyry 6. Sienite 7. Newer serpentine. The granite is the undermost, and the sienite the uppermost of the primitive formations. Granite is scarcely mixed with any other rock; but in gneiss, mica-slate, and clay-slate, there occur beds of old porphyry, primitive trap, primitive lime-stone, old serpentine, quartz rock. For that reason these rocks are said to constitute formations subordinate to gneiss, mica-slate, and clay-slate. Gypsum occurs in beds in mica-slate, and old flint-slate occurs in the same way in clay-slate. Hence they constitute formations subordinate to mica and clay-slate. Thus, besides the seven principal primi tive formations, there occur seven subordinate formations, interspersed through the second, third, and fourth formations; and topaz rock, which lies over gneiss and under clay-slate, must be added to the list: so that the primitive formations altogether amount to fifteen. If we suppose the nucleus of the earth to have been first formed, and the formations to have been afterwards deposited in succession upon this nucleus, it will follow that the lowest formation is the oldest, and that the formations are newer and newer according as they approach the surface. This supposition accounts for some of the names given to the primitive formations. That porphyry, for example, is considered as the oldest, which lies lowest down in the series of formations, and those formations of porphyry which lie nearer the surface are considered as newer. Granite, of course, according to this way of speaking, is the oldest formation of all, while the alluvial are the newest of all. The following Table exhibits a 'synoptical view of the primitive formations: → When a mountain is composed of layers of the same kind of stone, it is said to be stratified; but when the layers are of different kinds of stone, it is said to be composed of beds. |