THE ORIGIN AND DESCENT OF ROCKS. 403 



When a lava is cooled quickly, the commingled silicates solidify in 

 the diffused condition essentially as they were in the Hquid; for there is 

 no time for the silicate molecules of a like kind to come together, particle 

 by particle, in regular systematic order, as required in crystaUization. 

 The essential feature of crystallization is this systematic arrangement 

 of the molecules according to a definite plan, giving a specific crystal 

 form, as a cube, a hexagonal prism, etc. 



There are six (sometimes made seven) fundamental systems of crys- 

 tallization, and a multitude of variations of special form in each system. 

 The treatment of these forms belongs to mineralogy. 



In a thick viscid liquid, this systematic arrangement of molecules 

 into definite crystal forms takes place slowly, for the crystalline force 

 in the sihcates is far less energetic than that in water, which crystallizes 

 into ice with much rapidity and with great force. Because of this slow- 

 ness, the soHdification of the lava may catch the process of crystaUiza- 

 tion 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 

 crystaUine. In general the slower the growth the larger the crystals. 

 The solidification product may, therefore, range from a glass to a mass 

 of crystals; i.e., it may be (1) wholly glass, (2) a glassy matrix with 

 a few small crystals scattered through it, (3) a less abundant glassy 

 matrix with more and larger crystals, (4) a mere remnant of glass in 

 a mass of crystals, or (5) a mass of crystals with no glass. 



Successive stages of crystallization. — Since eruptions take place 

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

 in its hidden reservoir during the quiescent intervals between eruptions. 

 After a certain stage of partial crystallization has been reached during 

 such time of quiet, a renewal of eruption may take place and the whole 

 mass of lava be shifted into quite new conditions, and a second phase of 

 solidification may be superposed on the one already started. The rock 

 will then show two phases of crystaUization: (1) large crystals of the 

 kind or kinds most prone to develop in the given lava may have grown 

 during the first long stage of slow subterranean coohng, while the greater 

 part of the lava still remained liquid; and (2) small crystals or glass 

 may have developed when the more rapid cooling under the new con- 

 ditions took place. The result would be large crystals set in a matrix 

 of small crystals or of glass, a combination styled porphyritic. In such 

 cases the lava, in its later stages, carries the large crystals floating 

 throughout its mass, and is not a simple Hquid. 



