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Show 10 VOLCANIC AND PRIMARY ROCKS. [Ch.II, usua1 1 y con t am• so rne peculiar organic remain. s; as, for example, certam. speci.e s of shells and corals, or certam plants. . . TT z · c1 8 ,. o cantc ro tc • Besides these strata of aqueous or1gm, we find other rocks which are immediately recognized to be the pro d uc t sof fire , fwm their exact r. esemblance to those which have been produced in modern tlmes by volca~os, and thus we immediately establish two distinct orders of mmeral masses composing the crust of the globe-the sedimentary and the volcanic. Primary rocks. But if we investigate a large .portion of a continent which contains within it a lofty mountam range, we rarely fail to discover another class, very distinct from either of those above alluded to, and which we can neither assimilate to deposits such as are now accumulated in lakes or seas, nor to those generated by ordinary volcanic action. The class alluded to, consists of granite, granitic schist, roofing slate, and many other rocks, of a much more compact and crystalline texture than the sedimentary and volcanic divisions before mentioned. In the unstratified portion of these crystalline rocks, as in the granite for example, no organic fossil remains have ever been discovered, and only a few faint traces of them in some of the stratified masses of the same class; for we should state, that a considerable portion of these rocks are divided, not only into strata, but into laminre, so closely imitating the internal arrangement of well-known aqueous deposits, as to leave scarcely any reasonable doubt that they owe this part of their texture to similar causes. These remarkable formations have been called primitive, from being supposed to constitute the most ancient mineral productions known to us, and from a notion that they origi· nated before the earth was inhabited by living beings, and while yet the planet was in a nascent state. '!'heir high relative antiquity is indisputable; for in the oldest sedimentary strata, containing organic remains, we often meet with rounded pebbles of the older crystalline rocks, which must therefore lJave been consolidated before the derivative :;trata w~re formed out of Ch.II.] ORIGIN OF TilE PRIMARY ROCKS. 11 their ruins. They rise up from beneath the rocks of mechanical origin, entering into the structure of lofty mountains, so as to constitute, at the same time, the lowest and the most elevated portions of the crust of the globe. Origin of primary roclcs. Nothing strictly analogous to these ancient formations can now be seen in the progress of formation on the habitable surface of the earth, nothing, at least, within the range of human observation. The first speculators, however, in Geology, found no difficulty in explaining their origin, by supposing a former condition of the planet perfectly distinct from the present, when certain chemical processes were developed on a great scale, and whereby crystalline precipitates were formed, some more suddenly, in huge amorphous masses, such as granite; others by successive deposition and with a foliated and stratified structure, as in the rocks termed gneiss and mica-schist. A great part of these views have since been entirely abandoned, more especially with regard to the origin of granite, but it is interesting to trace the train of reasoning by which they were suggested. First, the stratified primitive rocks exhibited, as we before mentioned, well-defined marks of successive accumulation, analogous to those so common in ordinary subaqueous deposits. As the latter formations were found divisible into natural groups, characterized by certain peculiarities of mineral composition, so also were the primitive. In the next place, there were discovered, in many districts, certain members of the so-called primitive series, either alternating witli, or passing by intermediate gradations into rocks of a decidedly mechanical origin, containing traces of organic remains. From such gradual passage the aqueous origin of the stratified crystalline rocks was fairly inferred; and as we find in the different strata of subaqueous origin every gradation between a mechanical and a purely crystalline texture; between sand, for example, and saccharoid gypsum, the latter having, probably, been pre4 c1pitated originally in a ct·ystalline form, from water containing sulphate of lime in splution1 so it was imagined that, in a |