Northern Margin Of Boulder Batholith

Knopf (1957) has described a number of relatively minor occurrences of peralkaline and nepheline-bearing rocks along the northern boundary of the Boulder Batholith which he ascribes to limestone syntexis. One locality about 1.2 km southeast of Montana City comprises dykes of nepheline shonkinite and 'allied rocks' cutting a large roof pendant of limestone in granite or granodiorite. Shonkinite dykes up to 60 cm thick occur at Grizzly Gulch, southwest of Helena, and most extensively as dykes nearly 100 m thick cutting limestones near Austin.