22 GEOLOGY 



dark and more or less opaque. Lavas, too, are 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 a lava is cooled quickly, 

 the commingled silicates solidify in their diffused condition essen- 

 tially as they were in the liquid; for there is no time for the mole- 

 cules of one kind to come together in regular systematic order, as 

 is necessary to form crystals. The essential feature of crystalliza- 

 tion is this systematic arrangement of the molecules of a given 

 kind, according to a definite plan, giving a specific crystal form. 



In a thick viscid liquid, this systematic arrangement of mole- 

 cules into definite crystal forms takes place slowly, for the crystal- 

 line force in the silicates is far less energetic than that in water, 

 which crystallizes into ice rapidly and with great force. Because 

 of the slowness of crystallization, the solidification of the lava may 

 catch the process of crystallization at any stage. If the lava is 

 cooled quickly, the result is a glass; if less quickly, part glass and 

 part crystals; if slowly enough, all becomes crystalline. In general, 

 the slower the growth, the larger the crystals. 



The crystal forms of the different minerals are unlike. Thus a 

 crystal of quartz always takes the form of a six-sided pyramid 

 (Fig. 51), if nothing interferes with its growth. The crystals of 

 feldspar, mica, etc., take other forms (Figs. 49 and 50). All crystals 

 forms are grouped into six (sometimes made seven) fundamental 

 systems of crystallization, and there are a multitude of variations 

 of special form in each. The treatment of these forms belongs to 

 mineralogy. 



The first crystals to form from a liquid lava tend to assume per- 

 fect forms, but when many crystals are forming at the same time, 

 as is generally the case when lava solidifies, the crystals interfere 

 with one another's growth. The result is that they interlock in all 

 sorts of ways, and the forms of most of the crystals are very im- 

 perfect. 



Successive stages of crystallization. Since eruptions take place 

 intermittently, it is obvious that cooling of the lava may be in 

 progress beneath the surface during the intervals between eruption-. 

 After a certain stage of partial crystallization has been reached 



