CHAPTER XXV. 



On the rocks usually termed ' Primary ' Their relation to volcanic and sedimen- 

 tary formations The ' primary ' class divisible into stratified and unstratified 

 Unstratified rocks called Plutonic Granite veins Their various forms and 

 mineral composition Proofs of their igneous origin Granites of the same 

 character produced at successive eras Some of these newer than certain fos- 

 siliferous strata Difficulty of determining the age of particular granites 

 Distinction between the volcanic and the plutonic rocks Trappean rocks not 

 separable from the volcanic Passage from trap into granite Theory of the 

 origin of granite at every period from the earliest to the most recent. 



ON THE ROCKS COMMONLY CALLED PRIMARY. 



WE shall now treat of the class of rocks usually termed c pri- 

 mary,' a name which, as we shall afterwards show, is not always 

 applicable, since the formations so designated sometimes belong 

 to different epochs, and are not, in every case, more ancient 

 than the secondary strata. In general, however, this division 

 of rocks may justly be regarded as of higher antiquity than 

 the oldest secondary groups before described, and they may, 

 therefore, with propriety be spoken of in these concluding 

 chapters, for we have hitherto proceeded in our retrospective 

 survey of geological monuments from the newer to those of 

 more ancient date. 



In order to explain to the reader the relation which we con- 

 ceive the rocks termed ' primary' to bear to the tertiary and 

 secondary formations, we shall resume that general view of the 

 component parts of the earth's crust of which we gave a slight 

 sketch in the preliminary division of our subject in the 

 2nd chapter *. 



We there stated that sedimentary formations, containing 

 organic remains, occupy a large part of the surface of our con- 

 tinents, but that here and there volcanic rocks occur, breaking 

 through, alternating with, or covering the sedimentary deposits, 

 * See above, p. 8, 



