1818.] ON THE FORMATION OF ROCKS. 271 



Third Class. The origin doubtful, resembling a 

 little the second order of the first and second classes, but 

 the analogy neither direct nor positive, amounting only to 



probable conjecture. 



• 



First Order. Such rocks as probable conjecture 

 would incline to place in the Neptunian origin: 



Gneiss Clay Slate, 



Mica Slate, Primitive Limestone. 



Second Order. Such rocks as prob^^ble conjecture 

 would, incline to place in the volcanic origin: 



Hornblende, Sienite, 



Porphyry^ Granite. 



Greenstone, 



The origin of rocks may first be divided into the 

 known and the unknown. The two first classes contain 

 the known, and the third class the unknown. Farther ob- 

 servations may change their situation, and place a rock, 

 which is now in this unknown class, in the known class, 

 by which means the unknown class will diminish as our 

 positive knowledge increases, and in proportion as the 

 known class augments. 



The first clas§, or those rocks whose origin comes 

 within the limits of our positive knowledge, or can be 

 traced by positive analogy, divides itself into the Neptu- 

 nian and Volcanic, according as water or fire were in- 

 strumental in their formations. 



First Order, The Neptunian of Aqueous formation. 



