DOMAIN XII. VOLCANIC. 



in it they did not attain that degree ; 90 or 

 95 degrees may then be assumed as the 

 average heat of most volcanoes. 



" 2d. In this heat, many stones of the 

 argillaceous genus, as traps, hornblendes, 

 and argillites, undergo a change ; for they 

 alter their colour, become porous, assume 

 a porcelain grain, and consequently begin 

 to vitrify, as I have found on repeated 

 trials ; but they never flow in this heat, nor 

 consequently form a lava ; but bitumen will 

 flow in this heat, and even in one much 

 inferior, and be decomposed. If, there- 

 fore, the argillaceous stones be mixed with, 

 and drenched in bitumen, they will be 

 softened by it, and flow with it ; and where 

 the air, erupting both from them and the 

 decomposing bitumen, has most liberty to 

 escape, it will tumify, burst through the 

 liquid mass, and form cellular lava; but 

 where it is more compressed, less of it will 

 be disengaged, and the lava will be com- 

 pact, and resemble in some degree the ori- 

 ginal stone of which it is formed. 



" 3d. Stones of the siliceous genus un- 



