612 



GEOLOGY. 



LAVAS. 



Their nature. — In the chapter on the Origin and Descent of Rocks, 

 the nature of lavas and of the rocks derived from them has been dis- 

 cussed (Chapter VII). In view of prevalent misconceptions, it may be 

 repeated, for the sake of emphasis, that lavas are mutual solutions of 

 mineral matter in mineral matter, rather than simply melted rock. Into 

 this mutual solution there enter not only rock materials, but gases. 



Fig. 469. — Lobular form of lava-flow, ''Pahoehoe." (Button, U. S. Geol, Surv.) 



The distinction between mutual solutions and simple molten rock cannot 

 be rigorously made, but it is at least essential to know that the min- 

 erals do not necessarily crystallize from lavas in the order of their melt- 

 ing temperatures, or in any uniform order, but rather in the order in 

 which saturation of the several mineral constituents happens to be 

 reached in the given mutual solution. Thus quartz, which has a very 

 high melting-point, is often one of the last minerals to crystalhze. The 

 mutual solutions are exceedingly complex, embracing a wide range of 

 chemical substances, but the chief of them, as already stated, are sili- 

 cates of aluminum, potassium, sodium, calcium, magnesium, and iron, 

 with minor ingredients of nearly all known substances, in greater or 

 less proportion. The old idea of lavas as simply melted rock is not, 

 however, wholly to be abandoned. The mode of solidifying is often 



