52 



VOLCANOES 



As volcanoes so generally stand in or near the sea, and as the 

 lighter fragments, such as pumice, often drift for months upon the 

 water before they sink, while the finer dust is carried vast dis- 

 tances by the wind, it would naturally be expected that volcanic 

 materials should have a very wide distribution upon the sea- 

 bottom. Such, indeed, proves to be the case, and this kind of 

 material, laid down in the sea, has formed important rock-masses 



FlG. 16. — Pompeii, showing depth of volcanic accumulations. (Photograph by- 

 McAllister.) 



in nearly all the recorded ages of the earth's history. The exact 

 character of the rock formed in this fashion will be governed by 

 various circumstances, such as the fineness and abundance of the 

 material, whether it is showered into quiet waters or along a wave- 

 beaten coast, whether and in what proportion it is mingled with 

 sand or mud. When the volcanic ash preponderates, a tuff is 

 formed, very much like those which accumulate on land, but more 

 regularly stratified. 



The fragmental volcanic products, whether coarse or fine, retain 



