60 



spective levels from granite to clay-slate : wiiile the newer porphyry and 

 sienite are often laid over their summits, as though these two formations 

 had been deposited long after the production of the others ; an idea which 

 is still further strengthened by our meeting occasionally with a bed of 

 breccia, or pudding-stone, composed of fragments of the older or lower 

 rocks, capping the gneiss, granite, or other formation before the porphyry 

 or sienite has been deposited. 



The SECOND CLASS of rocks, or that which, when the number of coatings 

 is complete, lies immediately over the preceding, consists of gray-wacke 

 slate, and a peculiar kind of limestone, green-stone, and amygdaloid ; 

 together with subordinate masses of the proper primitive formations, sienite, 

 porphyry, and granite : as though some portions of these had become crys- 

 tallized after the rest, along with the next layers in succession, or had been 

 separated from the parent rocks by some early commotion. Gray-wacke, 

 which is a concrete term, denoting a conglomerate rock of a pecuhar kind 

 having a basis of clay-slate, and being studded or otherwise intersected 

 with portions of quartz, felspar, and scales of mica, may be exemplified by 

 what in Cornwall is called killas^ a far more euphonous word ; and hence 

 gray-wacke and gray-wacke slate may be distinguished by the terms amor- 

 phose and schistose killas. The Cornish killas lies directly over the granite 

 of that country, which possesses the character ascribed by Werner to 

 granite of the highest antiquity.* 



These formations, for the most part, irregularly alternate with each 

 other, instead of preserving one regular and successive order, as the dif- 

 ferent sets of the primitive formations do ; excepting that the limestone 

 appears usually undermost, and placed, as the basis of the rest, upon the 

 sienite or uppermost of the first class. It is in this second class of forma- 

 tions that petrifactions first make their appearance ; and it deserves par- 

 ticular attention that they are uniformly confined, both in the animal and 

 vegetable kingdoms, to those of the lowest links in the scale of organization; 

 and even among these to species which are at present altogether unknown, 

 and which appear therefore to be totally extinct. Thus the animal petri- 

 factions consist entirely of ammonites, mytihtes, unknown corals, and 

 other zoophytic worms ; and the vegetable petrifactions of reeds, ferns, 

 and other palm-like plants, mosses, and other cryptogamic productions, 

 which occupy the lowest part in the scale of vegetable fife, as zoophytic 

 worms do among animals. It is here, also, that carbonaceous matter, 

 which is chiefly of vegetable origin, first makes its appearance in any con- 

 siderable quantity. 



To this class of rocks, therefore, M. Werner has given the name of 

 TRANSITION FORMATIONS ; as believing them to have been produced while 

 the earth was in a state of transition from inorganic matter to organic life, 

 — from an uninhabited to an inhabited condition. The date of their for- 

 mation, however, is proved, even from their natural appearance, to have 

 been very remote ; since, as already observed, the whole of the petrifactions 

 which they contain consist of plants and animals, not only of the very 

 lowest species, but which now seem to be altogether extinct. 



The THIRD CLASS of rocks is denominated floetz, that is, flat or 

 HORIZONTAL, FORMATIONS, in consequencc of their usually appearing in 

 beds much more nearly horizontal than the preceding. They lie imme* 



* See Allan's remarks on the transition-rocks of Werner, in Thomson's Annals of Philos . 

 Tol. iii. p. 2f?- Compare with Jameson's deifinition of the same. Id. Feb. 1817, p. 17. 



