ORDER AND FORMATION OF ROCKS. 39 posed that the order of the rocks was pretty uniform; but many exceptions were soon discovered. The gniess, for example, is often wanting; and the granite is covered with mica-slate, hornblende rock, or with limestone or porphyry. One thing is worth your observation: as granite is the lowest, or foundation-rock, it is for the same reason the highest, and forms the summit of many of the highest mountains. EDWARD. I confess I cannot understand that; for, if it be the foundation of the other rocks, it ought to be covered and concealed by them. MRS. R. It would be so, undoubtedly, were all the rocks. placed horizontally, as you would lay one book over another on a table; but granite, gniess, mica, and clay-slate, hornblende rock, sienite, greenstone, and some of the others, are placed almost on end, so that the granite, which is the lowest at one end, is the highest at the other, as you will understand by placing a few books perpendicularly, and others resting on them, to represent the different rocks. 40 ORDER AND FORMATION OF ROCKS. EDWARD. But, if I were to continue to add book after book, each resting on the one before it, they would at last lie horizontally, and parallel with the table; is it so with rocks? MRS. R. Precisely so. The granite is generally perpendicular, or nearly so; and the gniess, and the other rocks that more immediately rest on it, are found at high angles; while the gray wacké, the conglomerates, and the sandstone, are at low angles; and the basalt, shale, coal, but more particularly the alluvial substances, gravel, sand, and clay, lie horizontally. CHRISTINA. See, here is a whole range of books, according to what you have just told us; the granite standing high and perpendicular, and the other rocks all following in their order. [See plate.] MRS. R. This will so far assist you to understand the po |