I91I.] IDDINGS— PROBLEMS IX PETROLOGY. 287 



I. Actual Mineral Composition of Igneous Rocks. 



Although the minerals constituting various rocks are their most 

 obvious features, aside from their general color and texture, and 

 have been the chief object of study by petrographers since the intro- 

 duction of microscopical methods of investigation, they still remain 

 among the most important problems before the petrologist. 



The exact composition, crystal characters and optical properties 

 of many of the minerals are well known. But some of the common- 

 est, such as the micas, amphiboles and aluminous pyroxenes, are not 

 perfectly understood chemically, and the relation between their com- 

 position and optical constants is not so definite that one may be 

 employed to determine the other, as is the case with the lime-soda- 

 feldspars. 



Moreover, the exact amounts of the component minerals in 

 various kinds of igneous rocks have not been determined, except 

 in a very few instances ; nor has the precise composition of those 

 minerals that occur in mixed crystals, that is, the principal ferro- 

 magnesian minerals, been determined in the vast majority of the 

 rocks described. 



There is, therefore, a great field of research, imperfectly culti- 

 vated, capable of yielding immediate returns of the first importance 

 for the solution of other problems connected with the mineral com- 

 position of these rocks. 



Similarly, more definite and specific study and description are 

 needed of the cr}-stal forms and arrangements of the mineral con- 

 stituents of igneous rocks than have heretofore appeared in petrog- 

 raphy, in order that the texture of various rocks may be clearly- 

 understood, since texture is a very definite exponent of physical 

 conditions that attended the crystallization of each igneous magma. 

 Up to the present time petrographers have been content with very 

 vague and incomplete descriptions of rock textures, as well as of 

 kinds and amounts of minerals composing various igneous rocks. 



The determination of the kinds and amounts of the minerals in 

 every rock leads to the problem of the formation of the minerals 

 in each instance, and a comparison of the mineral composition of 

 a rock with the chemical composition of the magma from which it 



