Abstract
A detailed account is given of the occurrence and the mineral and chemical composition of the granodiorites found in the Pend Oreille district, Bonner county, Idaho. The study of the minerals of which these rocks are composed brought out some facts of general application. The biotite and the chlorite have higher indices of refraction than are generally credited to these minerals in common igneous rocks. Comparison with the indices of refraction of the biotite and chlorite of many well-known igneous rocks showed, however, that the biotite and chlorite of these granodiorites are not unusual. The rocks show pronounced effects of hydrothermal and some effects of pneu-matolytic alteration. These processes are discussed in some detail, since it is thought that they have acted on many other granitic rocks.
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