Absarokite-Shoshonite-Banakite Series

Abstract

IN the region of the Yellowstone National Park there are igneous rocks of a peculiar type which are associated with the normal andesites and basalts of the region, but which differ from them mineralogically and chemically, and deserves a special classification. The study of these rocks has been carried on in connection with that of all of the igneous rocks of this region, and the present paper is an abstract of a chapter prepared for the report of work done in the Yellowstone National Park by the division of the U. S. Geological Survey under the charge of Mr. Arnold Hague. The rocks mentioned occur in a number of separate localities within this region, where it is apparent that their generic relations are with normal basalts and andesites, and in each locality the varieties having the peculiarities in question are genetically related to one another by differentiation. But all of these peculiar varieties in the region are not closely related to one another, for they are separate offshoots from distinct reser-voirs of magma, and were probably produced by similar pro-cesses of differentiation. For purposes of systematic descrip-tion they may be classified together in a series having certain chemical and mineralogical characteristics. They thus form classes of similar rocks (i. e., like phases of differentiation), that belong to separate, but similar, families of rocks (i. e., groups or series of genetically, hence generically, related differentiation