10 VOLCANIC AND PRIMARY ROCKS. [Ch. II. 



usually contain some peculiar organic remains ; as, for example, 

 certain species of shells and corals, or certain plants. 



Volcanic rocks. Besides these strata of aqueous origin, we 

 find other rocks which are immediately recognized to be the 

 products of fire, from their exact resemblance to those which 

 have been produced in modern times by volcanos, and thus 

 we immediately establish two distinct orders of mineral 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 mountain 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 consider- 

 able portion of these rocks are divided, not only into strata, 

 but into laminae, so closely imitating the internal arrangement 

 of well-known aqueous deposits, as to leave scarcely any rea- 

 sonable 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. Their 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 have been 

 consolidated before the derivative strata were formed out of 



