162 SANDSTONE PETRIFACTIONS. 



cept the carbon, can be resolved into gaseous bodies, 

 and even it, by combining with oxygen, becomes 

 gaseous also. In this way, therefore, the vegetable 

 matter may all escape, and the quartzy or siliceous 

 take its place. And if it be asked, whence the 

 carbon should receive the oxygen, to render it ga- 

 seous, or to form carbonic acid ? the answer is ob- 

 vious — from the water ; for water seems always 

 to be present during the petrifying process. 



Few, I believe, will now be startled at the idea 

 of sandstone being a chemical deposit. Our Pre- 

 sident's two ingenious papers, " On the Mineralogy 

 of the Pentland Hills," and " On Conglomerated 

 Rocks," published several years ago in the Memoirs 

 of this Society, have rendered it familiar to us, and 

 have, I think, proved that sandstone is, in some in- 

 stances at least, a chemical deposit. The Profes- 

 sor has there shewn, that what have been taken for 

 fragments of previously existing rocks cemented by 

 a basis, are in fact contemporaneously formed con- 

 cretions, as much as the masses of felspar, of quartz 

 and of mica in granite, or of the quartz itself in 

 granular quartz, and other crystalline rocks. I shall 

 not attempt to go over the ingenious arguments by 

 which this position is made out, and by which it is 

 shewn to be very highly probable, that grey-wacke 

 also and trap-tuff, and in mafiy cases sandstone, as 

 well as conglomerated rocks of the primitive class, 

 such as granite, gneiss, mica-slate, porphyry, and lime- 

 stone, have had a chemical origin, and are not, as 



