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Show 14 SUCCESSION OF STRATIFIED MASSES, [Ch.II. occasionally graduate into the secondary; accordingly, an attempt was made, when the classificati.on .of rocks was chiefly derived from mineral structure, to mst1tute an order called transition, the characters of which were intermediate between those of the primary and secondary formations. Some of the shales, for example, associated with these strata, often passed insensibly into clay slates, undistinguishable from those of the granitic series ; and it was often difficult to determine whether some of the compound rocks of this transition series, called greywacke, were of mechanical or chemical origin. The imbedded organic remains were rare, and sometimes nearly obliterated ; but by their aid the groups first ca1led transition were at length identified with rocks, in other countries, which had undergone much less alteration, and wherein shells and zoophytes were abundant. The term transition, however, was still retained, although no longer applicable in its original signification. It was now made to depend on the identity of certain species of organized fossils; yet reliance on mineral peculiarities was not fairly abandoned, as constituting part of the characters of the group. This circumstance became a fertile source of ambiguity and confusion; for although the species of the transition strata denoted a certain epoch, the intermediate state of mineral character gave no such indications, and ought never to have been made the basis of a chronological division of rocks. Order of succession of stratified masses. All the subaqueous strata which we before alluded to as overlying the primary, were at first called secondary; and when they had been found divisible into different groups, characterised by certain organic remains and mineral peculiarities, the relative position of these groups became a matter of high interest. It was soon found that the order of succession was never inverted, although the different formations were not coextensively distributed; so that, if there be four different formations, as a, b, c, d, in the annexed diagt·am (No. 1 ), which, in certain localities, may be seen in vertical superposition, the uppermost or newest of them, Ch. II.] CHARACTER OF TIU: TERTIARY STRATA, 15 a, will in other places be in contact with c) or with the lowest No.1. of the whole series, d, all the intermediate formations being absent. Tertiary formations. After some progress had been made in classifying the secondary rocks, and in assigning to each its relative place in a chronological series, another division ot sedimentary formations was established, called tertiary, as being of newer origin than the secondary. The fossil contents of the deposits belonging to this newly-instituted order are, upon the whole, very dissimilar from those of the· secondary rocks, not only all the species, but many of the most remarkable animal and vegetable forms, being distinct. The tertiary formations were also found to consist very generally of detached and isolated masses, surrounded on aU sides by primary and secondary rocks, and occupying a position, in reference to the latter, very like that of the waters of lakes, inland seas, and gulfs, in relation to a continent, and, like such waters, being often of great depth, though of limited area. The imbedded organic remains were chiefly those of marine animals, but with frequent intermixtures of terrestrial and freshwater species so rarely found among the secondary fossils. Frequently there was evidence of the deposits having been purely lacustrine, a circumstance which has never yet been clearly ascertained in regard to any secondary group. We shall consider more particularly, in the next chapter, how far this distinction of rocks into secondary and tertiary is founded in nature, and in what relation these two orders of mineral masses may be supposed to stand to each other. But before we offer any general views of this kind, it may be useful to present the reader with a succinct sketch of the principal |
