VOLCANIC ROCKS. 27 



stance that the plutonic rocks consolidate deep 

 in the earth, while the volcanic rocks consolidate 

 under the pressure of the atmosphere, or near to 

 the surfa 



The principal types of volcanic rocks are 

 named Rhyolit* Andesites and Ba- 



salts. The basalt has been supposed to be the 



• formed; and to have CO me from a greater 



th than the other-, being commonly the 

 den- it frequently rests 



upon andesites and rhyolites. These rocks have 



n repeated several times in succession in the 

 history of the earth. Rhy aind in the 



old pre-Cambrian rocks of Wales; andesites in 



the Cambrian rocks of the Lake district, and the 



Old Red Sandstone of Scotland; while in the 



later Coal Measures there were countless out- 

 bur- tsalt. The volcanic rocks Of the Ter- 

 tiary period in Britain are a repetition of those of 



the Primary period, basalt- ding andesites 



and rh\ 



There IS at the present day something like a 

 ai distribution of the different volcanic 

 rocks. The VOlcanOS of the Andes pour out the 

 rock named andesite. The volcanos of Southern 

 Italy give out varieties of basalt. Metals are 

 very rarely associated with volcanic eruptions, 

 though an appreciable quantity of silver has been 

 found in volcanic ash of eruptions in Chili. 



Chemical and mineral composition alike sug- 

 gest the closest relation between the deep-seated 

 crystalline rocks and those which flow from vol- 

 canos. The plutonic granite appears to become 

 the volcanic rhyolite. Plutonic syenite and dio- 

 rite on reaching the surface appear to become 

 andesite. And the rock which cooling under 



