192 IGNEOUS ROCKS 



place by a process of crystallization, and this requires time. Hence, 

 very rapid cooling results in a glass, but the microscope reveals 

 the incipient stages of crystallization in many of even the glassy 

 rocks. A somewhat slower rate of solidification produces a 

 cryptocrystalline rock, and successively slower rates bring about 

 the porphyritic, microcrystalline, and granitoid textures. Large 

 crystals form slowly, and other things being equal, the larger 

 the component crystals of a rock, the more slowly has it 

 consolidated. 



Pressure is of importance in preventing the rapid escape of the 

 vapours and gases contained in the molten mass, and hence frothy, 

 scoriaceous, and vesicular textures cannot be produced under high 

 pressures. Pressure is also believed to be necessary for the forma- 

 tion of many phenocrysts in porphyritic rocks. 



The mineralizers, such as steam, hydrochloric acid, and other 

 vapours, determine the crystallization of many minerals, which 

 refuse to crystallize in the absence of such vapours. Variations in 

 the quantity of mineralizers present in different parts of the same 

 mass, occasion corresponding differences in the local textures. 

 The well-known Obsidian Cliff, in the Yellowstone National Park, 

 is formed by a great sheet of igneous rock, made up of alternating 

 layers of glassy and microcrystalline rock, a difference which is 

 referred to varying proportions of mineralizers present in different 

 parts of the molten mass. 



It must not be supposed that a molten magma consists merely 

 of a number of fused minerals, mechanically mixed together and 

 having no effect upon one another. If such were the case, the 

 minerals in cooling should all crystallize in the order of their fusi- 

 bility, the least fusible forming first, and the most fusible last. 

 This is not what we find, and many facts which cannot be discussed 

 here have led petrographers to the belief that a molten magma 

 is a solution of certain compounds in others, and that crystalliza- 

 tion occurs in the order of solubility, as the point of saturation for 

 particular compounds is successively reached by the cooling mass. 



Similar phenomena may be observed among the metals. If 

 strips of copper be thrown into a vessel of melted tin, the latter 



