340 ON THE FORMATION OF ROCKS. [Aug-ast, 



miiiated : for this reason the analogy is not direct, and 

 the origin must remain doubtful. 



Porphyry, in its structure antl external appearance, 

 resembicfe much some lavas, more particularly those 

 of the oldest kind, where the asperity has been worn 

 off, and softened by time : but in its mode of stratifica- 

 tion and relative situation, it is similar to the Neptu- 

 nian origin, therefore the analogy is not direct aiid the 

 origin must remain doubtful. 



Granite. Tliere ai*e two species of Granite ; the one 

 in large grains, which occasionally alternates with 

 gneiss, and contains many of the valuable specimens^ 

 of minerals, such as the emerald, cymophane, tour- 

 maline, &c. &c.; andtlie other, a middling grained Gra- 

 nite, often with much <|u;irtz in it, occurring under all 

 otlier rocks, in large lields, witlioiit any well defined 

 stratification, hut divided often by vertical fissures. 

 This last is the Granite of wliich we are speaking ; it 

 has more resemblance to some of the feldspatic lavas, 

 til an it has to any rock known to be of Neptunian ori- 

 c;in. It likewise approaches the volcanic, in relative 

 siuiation, without any regular ^^stratification. Yet the 

 resemblance does not appear sulTiciently strong to 

 amount to direct analoi^y, and we must therefore re- 

 rf-iiii in doubt as to the nature of its origin. This Gra.- 

 nite is the lowest rock in the arrangement of tlie globe; 

 tlirough which wc liave never penetrated; and beyond 

 \ihich we know nothino:. Is it the nucleus of the earth, 

 ^rom wliK-?), and on vrhich, all clianges and formations 



