66 



CLASSIFICATION OF PRIMARY ROCKS. 



1 shall now proceed to describe the rocks denominated primary, 

 without any reference to theory ; and shall propose an arrangement 

 of them that will, I trust be found conformable to the present state of 

 the science. 



Primary rocks are composed chiefly of the hard minerals, quartz, 

 felspar, and hornblende ; the minerals, mica and talc, are dissemina- 

 ted in smaller proportions, and limestone and serpentine occur in 

 beds or masses, but less frequently than the above-named minerals. 

 If we refer the slate rocks to the transition class, the few simple min- 

 erals here enumerated constitute nearly the whole of the mountains 

 denominated primary. 



The structure of primary rocks is crystalline ; they form the cen- 

 tral parts of the most elevated mountains chains, and they occur also 

 at the lowest depths that have yet been explored, and are, hence, 

 believed to be the most ancient of rock formations. 



Werner has enumerated fourteen primary rocks ; but as some of 

 these have hitherto been found in only one place, it appears improp- 

 er to consider them as distinct orders, unless we arrange every varie- 

 ty of rock in the same manner, and increase the number of orders 

 indefinitely. 



The following arrangement of primary rocks includes only three 

 principal rocks as primary — granite, gneiss, and mica slate, which 

 are nearly allied to granite, and form an incrustation over it: these 

 never contain organic remains, and they have, rarely, been observed 

 lying over other rocks in which such remains are found. It com- 

 prises also the rocks which are sometimes found imbedded in gran- 

 ite, gneiss, and mica slate, and are regarded as subordinate formations. 



Class. I. 



Principal Rocks denominated Primary. 



1 . Granite, comprising all the varieties of this rock, and small- 



grained granite passing into porphyry, the Eurite of the 

 French geologists, primitive porphyry of the Germans. 



2. Gneiss or slaty granite. 



3. Mica slate. 



Subordinate Rocks wJiich occur among Primary. 



Hornblende rock. 

 Serpentine. 

 Crystalline limestone. 

 Quartz rock. 



Some of these subordinate rocks occur also among rocks of the 

 transition class. 



The three principal rocks of the primary class, — granite, gneiss, 

 and mica slate, — might, with propriety, be regarded as belonging to 

 one formation. They are, composed essentially, of the same min- 



