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THE VOLCANIC ROCKS OF THE ANDES.
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development of the rock. Consequently he concludes that upon purely theoretical grounds a chemical classification would be the most desirable. But from the present condition of our knowledge this would be impracticable.

Moreover, he observes, that while the chemical analysis of an unaltered rock furnishes us with the proportions in which the elements existed in the molten magma, it is very probable that rocks of like chemical composition may have been derived from magmas consisting of quite different silicates (that is, possessing different molecular constitutions). And this he suggests is one of the reasons why eruptive rocks with corresponding analyses can exhibit quite different mineralogical compositions. In a foot note he observes that another cause for this phenomenon may lie in differences in the process of solidification, which may affect the rearrangement of the compounds originally in the magma. To this extent the mineralogical composition is dependent on the genesis of eruptive rocks, which he considers essentially the same as their geological mode of occurrence.

Since we are not in a position to infer the original molecular constitution of a magma from its chemical analysis, he does not consider a chemical basis of classification applicable. Nevertheless he states that a comparison of the chemical composition of the andesitic lavas with their mineralogical composition shows that certain differences of chemical composition go hand in hand with others of mineral development, and with these are also connected modifications of structure.

The mineralogical features are the most pronounced, and are therefore selected as a basis of classification. The first subdivision is based on the presence or absence of quartz, and the groups become andesites and dacites. They are not, however, distinctly separated from one another, being connected by gradual transitions. But this grouping, as a purely mineralogical one, fails, as he himself points out, in cases where quartz has not crystallized, as in certain dacitic glasses.

In the further subdivision of these groups the ferromagnesian minerals are employed as distinguishing characteristics, and the

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