244 



MINERALOGY 



blende; quartz and tourmaline. Ternary and quaternary, eutectics 

 are also possible. Eutectic consolidation will at times represent a 



second generation 

 of small crystals, 

 surrounding the 

 porphyritically de- 

 veloped crystals of 

 the first generation. 

 Minerals which 

 are crystallized 

 from molten mag- 

 mas are usually an- 

 hydrous or contain 

 small amounts of 

 water, as the micas, 

 and are termed pri- 

 mary minerals, to 



FIG. 369. Porphyritic Feldspar Crystals in a Fine 

 Grained Crystalline Ground Mass. 



distinguish them 

 from secondary min- 

 erals, which have resulted from water solutions, the components of 

 whioh have resulted from the breaking down of other minerals. 



The order of crystallization of the primary minerals from magmas 

 is not rigidly fixed, 

 but may vary accord- 

 ing to the composition 

 of the magma, as the 

 curve of separation of 

 any individual species 

 is the result of the 

 lowering of the freez- 

 ing point of that in- 

 dividual by all the 

 other components of 

 the system, and these 

 may have a greater 

 effect on one than on 

 another. The order 

 of crystallization and 

 the relation of the 

 crystals of one species to those of an another species will depend 

 upon and reflect magmatic peculiarities. There are, however, 



FIG. 370. Section of a Mieropegmatite enlarged, 

 showing Eutectic Structure. 



