352 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



" The anomalies in the Esia trachyte formation might perhaps 

 " also be explained by movements of the fluid matter in the 

 " interior, or by alterations in the relief forms of the surface of 

 " separation just mentioned." The method of explaining these 

 anomalies by movements in the fluid matter existing beneath the 

 crust would appear to be the most reasonable, and I shall endea- 

 vour as briefly as possible to refer to it more minutely. 



We have already seen in referring to the cause of earthquakes 

 and volcanic eruptions, that it is uot impossible that movements 

 take place in the interior of the earth similar to tides in the ocean 

 on its surface. We shall suppose the two lines within the dark 

 coloured crust in the subjoined figure (1) to represent respectively 



Fig. 1. 

 the normal limits towards the interior of the acid, and the more 

 basic melted matter, which in the earliest periods as now, may be 

 supposed to have graduated into each other and yielded all the 

 varieties of eruptive rocks now visible on the surface. We shall 



