78 



GEOLOGY. 



Fig. 30. 



that which is at the surface, owing to rapidity of cool- 

 ing and the action of the air, is porous, and has the name 

 of scoria. What is called pumice, it is supposed, was 

 cooled by being thrown into water. Though its mineral 

 constitution is the same with the hard, compact lava, it 

 will float on w r ater from its great porousness. It is used 

 for polishing various substances, as wood, ivory, mar- 

 ble, glass, parchment, etc. Obsidian and pitchstone are 

 vitreous or glassy lavas, the former resembling glass 

 more than the latter, which receives its name from its 

 pitchy lustre. The lavas are very much like the rocks 

 of the trap family, a fact which is of much significance 

 in relation to the formation of the latter, as you will see 

 in a future stage of our investigation. 



There are other rocks limestone, serpentine, etc.; 

 but these I have spoken of sufficiently before. 



