IGNEOUS ROCKN 251 



usually mixtures of several silicates, while manufactured glasses 

 consist of only one, or at most a few. Furnace slag is essentially an 

 artificial lava. 



Solidification and crystallization. When lava is cooled quickly, 

 its components solidify essentially as they were in the liquid; for 

 there is no time for the molecules of one kind to come together in 

 regular systematic order, as is necessary to form crystals. In a 

 thick viscid liquid, the arrangement of molecules into definite 

 crystal forms takes place slowly. Because of this slowness, the 

 solidification of the lava may catch the process of crystallization at 

 any stage. This is why some igneous rocks are glassy (cooled 

 quickly), some partly glassy (cooled less quickly), and some wholly 

 crystalline. In general, the slower their growth, the larger the crystals. 



Stages of crystallization. Eruptions take place intermittently, 

 and the lava beneath the surface may be cooling during the inter- 

 vals between eruptions. After a certain stage of partial crystalliza- 

 tion has been reached during such time of quiet, a new eruption may 

 shift the whole mass of lava into new surroundings, and a second phase 

 of solidification may be added to the first. The rock may then show 

 two phases of crystallization: (i) large crystals of the kind or kinds 

 'developed during the first stage of slow subterranean cooling; and 

 (2) small crystals or glass developed during the more rapid cooling 

 of the second stage. The result is large crystals set in a matrix of 

 small crystals or of glass. This is perhaps one way in which por- 

 phyry is formed. 



Composition of Igneous Rocks 



Nearly all the chemical elements are found in igneous rocks, 

 though but few of them are abundant. These few are regarded a> 

 the essential constituents, while the rarer substances are regarded as 

 incidental. The relative amounts of the more abundant elements 

 in the crust of the earth, as nearly as now known, are shown in the 

 following table: 



Per cent in the 



