Abstract
A partly eroded laccolith of the Shonkin Sag type about 450 feet thick and 2 miles across forms a prominent flat-topped butte a few miles east of Boxelder, Montana. An upper layer of cliff-forming syenite about 120 feet thick grades downward to a coarse, friable shonkinite about 175 feet thick. Below this are a cliff-forming shonkinite about 40 feet thick and a friable, ribboned shonkinite about 50 feet thick, which has horizontal layers of syenite. The layers (ribbons) of syenite are thin and closely spaced near the base of the laccolith and become thicker and more widely spaced upward. Narrow irregular dikes of aplitic syenite cut the syenite and shonkinite. The coarse, friable shonkinite contains syenitic segregations.
Qualitatively, the rocks are much alike mineralogically. The shonkinite contains augite, olivine, biotite, sanidine, plagioclase, and analcime. The syenites lack olivine and contain aegirite. The aplitic syenite lacks plagioclase.
The main part of the laccolith is believed to have been injected as a shonkinitic magma and to have differentiated in place by crystal settling to form the upper syenite and most of the underlying shonkinite. The ribbons of syenite and the dikes of aplitic syenite were formed by local segregations of the residual liquid from the crystallizing shonkinite. When the first magma had largely crystallized, a second injection of magma formed the brim shonkinite.
