Little Rocky Mountains

Rising to about 1000 m above the surrounding monotonous plains the Little Rocky Mountains contain a core of granite and syenite porphyry, and schists surrounded by sedimentary rocks ranging from Cambrian to Cretaceous in age. The granites and syenites are leucocratic, with occasionally a little biotite or hornblende, but one specimen described by Weed and Pirsson (1896a, p. 418) from north of Mission Butte contained a little aegirine. On the southwest side of the mountains at the margin of the granite-syenite mass just north of Landusky a phonolite was sampled (op. cit., p. 419) consisting of alkali feldspar phenocrysts up to 1 cm long in a groundmass of alkali feldspar, aegirine-augite zoned to aegirine and nepheline.