OCR Text |
Show 36 DETERMINATION OF TilE [Ch.IV. masses h as b een dl'sttlrbed ' at different periods, by convulsio.n s of extraord m. ary v1'olence' as in the Alps and . o.t her mou..n tam-ous d1. str1·c t s, there are instances where the or1gmal position of strata J1 as b ee n re"•e t·sed '· but such exceptions are rare, and are usua1 1 y on a smca ll scale ' and • an experienced ob• server •c an generally ascertain the true relatiOns of the . roc~.s m questiOn, by examining some adjoining districts ~here the derangement has been less extensive. In regard to volcanic formations, if we find ~ stratu~ of tuff or ejected matter, or a stream of lava coven~g sedunenta? strata, we may infer, with confidence, that the Igneous rock 1s the more recent; but, o~ the other hand, the superposition of aqueous deposits to a volcanic ma~s does not a~ways prove the former to be of newer origin. If, mdeed, we discover strata of tuff with imbedded shells, or, as in the Vicentine and other places, rolled blocks of lava with adhering shells and corals, we may then be sure that these masses of volcanic origin covered the bottom of the sea, before the superincumbent strata were thrown down. But as lava rises from below, and does not always reach the surface, it may sometimes peneh·ate a certain number of strata' and then cool down, so as to constitute a . solid mass of newer origin, although inferior in position. It is} for the most part, by the passage of veins proceeding from such igneous rocks through contiguous sedimentary strata, or by such hardening and other alteration of the overlying bed, as might be expected to result from contact with a heated mass, that we are enabled to decide whether the volcanic matter was previously consolidated, or subsequently introduced. P·roofs by incl-uded fragments of older roclcs. A geologist is sometimes at a loss, after investigating a district composed of two distinct formations, to determine the relative ages of each, from want of sections exhibiting their superposition. In such cases, another kind of evidence, of a character no less conclusive, can sometimes be obtained. One group of strata has frequently been derived from the degrada~ Ch.IV.] RELATIVE AGES OF ROCKS. 37 tion of another in the immediate neighbourhood, and may be observed to include within it fragments of such older rocks. Thus, for example, we may find chalk with :flints, and in another part of the same country, a distinct series, consisting of alternations of clay, sand, and pebbles. If some of these pebbles consist of :flints, with silicified fossil shells of the same species as those in the chalk, we may confidently infer, that the chalk is the oldest of the two formations. We remarked in the second chapter, that some granite must have existed before the most ancient of our secondary rocks, because some of the latter contain rounded pebbles of granite. But for the existence of such evidence, we might not have felt assured that all the granite which we see had not been protruded from below in a state of fusion, subsequently to the origin of the secondary strata. Proofs of contemporaneous origin de1'ived from mineral characters. When we l1ave established the relative age of two formations in a given place, by direct superposition, or by other evidence, a far more difficult task i·emains, to trace the continuity of the same formation, or, in other cases, to find means of referring detached groups of rocks to_ a contemporaneous origin. Such identifications in age are chiefly derivable from two sources-mineral character and organic contents; but the utmost skill and caution are required in the application of such tests, for scarcely any general rules can be laid down respecting either, that do not admit of important exceptions. If, at certain periods of the past, rocks of peculiar mineral composition had been precipitated simultaneously upon the :floor of an' universal ocean,' so as to invest the whole earth in a succession of concentric coats, the determination of relative dates in geology might have been a matter of the greatest simplicity. To explain, indeed, the phenomenon would have been difficult, or rather impossible, as such appearances would have implied a former state of the globe, without any analogy |