This rock name, introduced by Weed and Pirsson in 1895, is derived from Shonkin, the Indian name for the Highwood Mountains of Montana. It is the only shonkinite in Maine. The modern classification of igneous rocks is more systematic, assigning names on the basis of percentages of the minerals quartz, alkali feldspar, and plagioclase feldspar. By this system, the Lincoln Sill rock, which lacks quartz and contains little plagioclase, would be called an alkali feldspar syenite. A rock with more quartz and plagioclase would be granite.
Granite is common.
It forms from melting of rock at moderate depths in the earth's crust. Shonkinite, on the other hand, is rare, and probably originates by melting much deeper in the lower crust or in the earth's mantle. The boulders on Burnt Island have been broken loose from the bedrock and carried by glacier ice to where they now rest. Since this kind of rock is not part of the bedrock of Burnt Island, it must have been carried for some distance. Bedrock maps show where various rock types are found in bedrock.