WRIGHT! CRYSTALS AND CRYSTAL FORCES 331 



namics and crystallography. The recognition and delineation 

 of the fields of application of thermodynamics and of crystallog- 

 raphy in problems of crystal equilibria is an essential step in 

 the solution of such problems ; but before this can be done satis- 

 factorily more quantitative data on interatomic crystal forces 

 are required. 



The rock-making minerals. As a good example of the kind 

 of problem in which crystal configurations of certain types per- 

 sist over a great range of conditions of formation, the rock- 

 making minerals may be cited. The most remarkable fact in 

 petrology is the relatively few rock-making mineral species, 

 especially in igneous rocks. These few minerals persist the 

 world over and constitute the major part of the rocks of the 

 earth's crust; and yet their number can be counted on the 

 fingers of the two hands — thus quartz, the feldspars, micas, 

 amphiboles, pyroxenes, nephelite, and calcite predominate; mag- 

 netite, zircon, apatite are also common, but their total amount 

 is small. This persistency of a few mineral species, notwith- 

 standing great diversity in conditions of formation and in chemi- 

 cal composition, is fundamental. In magma solutions such fac- 

 tors as temperature, pressure, solubility relations, rates of reac- 

 tion, change in composition by virtue of escape of volatile com- 

 ponents, and crystal nucleation enter the problem; but it appears 

 that, in spite of the great diversity possible in such complex chemi- 

 cal systems, the crystal groupings of the chemical elements, 

 which do result, are exceedingly few in number. These group- 

 ings represent, of course, the resultant of all the forces involved; 

 the problem is in part to ascertain the relative importance of 

 these several factors. It may be inferred that possibly the domi- 

 nating factor in the crystallization of a magma is the stability 

 of certain crystal types or configurations and that these assert 

 themselves notwithstanding tendencies toward other groupings 

 which thermodynamically are more stable. Of crystallographic 

 interatomic forces we know but little. The facts of observation 

 are, however, too patent to be disregarded in any consideration 

 of rock genesis, and are here cited as the kind of problem in 

 which a better understanding of crystal forces and of crystals is 



