Intruded into the Fort Union sedimentary formations is a remarkable assemblage of igneous rocks. In the southern part of the area, granites, many phases of diorite, gabbro, and peridotite occur as stocks, laccoliths, sills, and countless dikes. In the northern part of the area, super-alkaline rocks occur in profusion in laccoliths, sills, dikes, and three volcanic necks, as well as in a small diorite stock. No effusive rocks occur in the area. The sediments of the northern area have been intensely folded along north and south axes. In the southern area the Fort Union beds form a broad flat syncline plunging gently northward. Where pierced by the diorite stock the beds are arched and dip away on all sides. More than 20 different types of igneous rocks have been determined here. Many chemical analyses, including some mineral analyses, are presented. The rock analyses disclose one common feature—a richness in barium and strontium—pointing to a common magma. Pegmatites are absent. Pneumatolytic agents are not in evidence. The magma was evidently dry. Lateral compression evidently attended emplacement of the intrusives.

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