28 HlSTOR'i OF 



CHAP. V. 



OF FOSSIL- SHELLS, AND OTHER EXTKANEOUS FOSSILS. 



We may affirm of Mr Buffon, that which has been said at 

 tlie chemists of old; though he may have failed in attaining his 



through all the succeeding classes of rocks. This evidence of the vast dimi 

 nution of the volume of water which stood so high over the whole earth, is 

 assumed to be perfectly satisfactory, although we can form no correct idea 

 of what has become of it. By the earliest separations from the chaotic nias«. 

 which are discoverable In the crust of the globe, was formed a class of rocks, 

 which are therefore termed primitive rocl;s. The circumstances which mark 

 the high antiquity of these rocks are, that they form the fundamental rock of 

 the other classes. Having been formed in the uninhabitable state of the 

 globe, they contain no petrifactions, and, excepting the small portion which 

 sometimes acctmpany those which will be next mentioned, they contain no 

 mechanical deposites, but are, throughout, pure chemical productions. 

 Small portions of carbonaceous matter, occur only in the newer members of 

 the cla.ss. Before the summits of the mountains appeared above the level of 

 the ocean, and before the creation of vegetables and animals, a rising of the 

 waters is supposed to have taken place, during which, that class of rocks 

 which are said to be of the second formation was deposited The rocks »f 

 this formation are clay, porphyry, pearl stone porphyry, obsidian porphyry, 

 seinite, and pitchstone ; they exhibit very few mechanical depositions, are 

 of complete chemical formation, and contain little or no carbonaceous mat- 

 ter, and never any petrifactions. On the appearance of land, or during the 

 transition of the earth from Its chaotic to its habitable state, rocks whicli 

 from this circumstance are denominated transition rocks were formed. In 

 these rocks, the first slight traces of petrifaction, and of mechanical de- 

 positions, are to be found. As the former class of rocks were purely of che. 

 mical formation, so the contents of these are chiefly chemical productions, 

 mingled with a small proportion of mechanical depositions ; to explain the 

 cause of the mixture, we are referred to the period of their formation, tha 

 at which the summits of the primitive mountains just appeared above the 

 waters, when, by the attraction excited by the motion of the waves, and 

 (vhich we are reminded extended to no great depth, particles of the original 

 mountains were worn oft' and deposited. As the height of the level ot tha 

 ocean diminished, so would the surface on which its waves acted increase, 

 and of course the number of mechanical depositions. Hence, these are much 

 more abundant in the rocks of the next formation, which are denominated 

 fiostx roclxs, (m account of their being generally disposed in horizontal or 

 flat strata. In these, petrifactions are very abundantly found, having beeu 

 formed whilst vegetables and animals existed in great numbers. These rocks 

 are generally of very wide extent, and commonly placed at the foot of primi. 

 tive mountains ; they are seldom of a very great height, from whence it may 

 be inferred, that the water had considerably subsided at the time of theil 

 formation, and did not then cover the whole face of the earth. Countries 

 wimposed of these rocks are not so rugged in their appearance, nor so marked 



