191 1.] IDDINGS— PROBLEMS IX PETROLOGY. 289 



Table I., under favorable conditions should form the mineral com- 

 pounds in the proportions shown in column 2. In this there are 13 

 per cent, of quartz, and 17 per cent, of orthoclase, together making 

 30 per cent, of the whole. And this amount of quartz is the least 

 amount of free silica capable of separating from a solution of such 

 a chemical composition, assuming that the minerals formed are those 

 known to occur in igneous rocks. A magma of this chemical com- 

 position commonly crystallizes as a pyroxene-andesite composed, so 

 far as the microscope can determine, of lime-soda-feldspar, pyrox- 

 ene, and magnetite, with no visible quartz or orthoclase. And, yet, 

 from the chemical analysis of the rock there should be 30 per cent, 

 of these compounds. 



The orthoclase molecules may be readily imagined in solid solu- 

 tions within the lime-soda-feldspars, although in coarsely crystal- 

 lized forms of such a magma, diorite, orthoclase crystals appear as 

 independent individuals. It has been shown in the Geophysical 

 Laboratory of the Carnegie Institution, that orthoclase and anor- 

 thite molecules form homogeneous mixed crystals when melted 

 together and cooled in an open crucible. The disappearance of 17 

 per cent, of orthoclase in this particular andesite is, therefore, due 

 to the conditions of solidification of the rock. The non-appearance 

 of the quartz may be explained in part by its existence in solid solu- 

 tion in other minerals of which, however, we have not sufficient 

 evidence at present ; or it may occur in minute crystals mistaken for 

 andesine feldspar, since the optical properties of the two that may be 

 recognized in minute crystals, are almost identical. In coarser 

 grained forms of chemically similar magmas the quartz appears, but 

 the conditions attending crystallization in the contrasted cases may 

 favor its disappearance through solid solution in one instance, and 

 its separation as quartz crystals in the other. 



In this connection it is to be pointed out that the apparent actual 

 mineral composition of certain igneous rocks may not be the real 

 mineral composition by as much as 30 per cent, of the whole. For 

 the occult minerals in solid solution are as much a part of the rock 

 as though visible. Moreover, the percentages assigned to the min- 

 erals that are seen must be in error by the amounts of the occult 



