Skalkaho
Published information regarding the Skalkaho intrusion is sparse. It is an elongate complex of about 7x2 km consisting of pyroxenites, syenite and pegmatitic mafic rocks, and is intruded by a variety of alkaline dykes.
Published information regarding the Skalkaho intrusion is sparse. It is an elongate complex of about 7x2 km consisting of pyroxenites, syenite and pegmatitic mafic rocks, and is intruded by a variety of alkaline dykes.
Lying just south of the Canadian border the Sweet Grass Hills comprise three buttes, East, West and Middle, around which Cretaceous and Jurassic sediments are domed and cut by igneous intrusions.
The alkaline igneous rocks of the Bearpaw Mountains are the most extensive of the central Montana alkaline igneous province, and comprise a considerable thickness of lavas and pyroclastic rocks together with numerous stocks, laccoliths, sills and dykes, including the Rocky Boy stock (No.
The Rocky Boy stock is the largest and most complex of the numerous intrusions lying along the central, east-west-trending arch of the Bearpaw Mountains (No. 25).
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.
A number of diatremes extend from Haystack Butte (described briefly in No. 29), northeast of the Highwood Mountains, and Eagle Buttes, close to the Missouri River, eastwards for 160 km through the Missouri River Breaks to the Little Rocky Mountains.
The igneous rocks of the Highwood Mountains occupy an area of about 50x50 km and represent the deeply eroded remnants of a volcanic centre of flows, a dyke swarm, stocks and laccoliths, one of the last being Shonkin Sag which is described independently (No. 30).
Shonkin Sag is the most studied of the Highwood Mountains laccoliths, because it was considered to be an outstanding example of the differentiation in place of an undersaturated alkaline magma.
The northern part of the Big Belt Mountains (or Range) is chiefly underlain by somewhat potassic basic volcanics about 1000 m thick, the Adel Mountain volcanics, which lie unconformably on Cretaceous sediments.
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.