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were erupted in early Tertiary or late Cretaceous time in most cases, but their exact date is not known. Owing to extensive erosion the extrusive forms of these rocks, if they ever reached the surface, have been entirely removed.

Mr. Lindgren observes in the first paper cited that the rocks of this region appear to be more varied in chemical composition than the series usually found in the Great Basin; magmas rich in potassium are frequent, crystallizing as trachytes; often they are very basic, and contain much sodium, resulting in the abundant separation of such minerals as nepheline, sodalite and analcite.

The more or less acid rocks in the Little Belt mountains and at various points in front of the main range, west of Fort Benton, constitute dacites, hornblende-andesites, and diorites. Similar rocks also occur in the Moccasin mountains. They vary much in structure and composition, and form a natural group. The prevalent habit is porphyritic, but there appears to be a continuous series of transitions from porphyritic to fine granular rocks. The phenocrysts are feldspar and hornblende, and sometimes quartz and mica. The porphyritical feldspars are in part orthoclase in varying quantities, and there is reason to believe that these rocks pass by gradual transitions into trachytic and rhyolitic forms.

Those varieties free from phenocrysts of orthoclase and quartz grade into medium grained diorite, analogous to Stelzner's "Andendiorit," which contain besides plagioclase, hornblende and biotite, a little orthoclase and quartz as the last minerals to crystallize.

Of the more basic rocks, a part are syenites and trachytes, and a part basalts. The syenites which form dikes consist principally of orthoclase, plagioclase, biotite and a pyroxene, probably malacolite. They are called augite-syenites. The syenite from near Dry Fork, Little Belt mountains, contains, in addition to these minerals, allotriomorphic grains of an isotropic substance, probably sodalite. The rock contains 5.50 per cent. of K₂O, and 4.14 per cent. of Na₂O. The augite-syenite from the Highwood mountains is coarsely granular, and contains 5.66 per cent. of K₂O and 7.88 per cent. of Na₂O. This syenite is surrounded by trachytic and basaltic dikes; and in one case a dike of syenite was seen cutting one of the basaltic dikes.

The syenite from Square Butte at the northern end of the Highwood mountain is characterized by a noticeable percentage of sodalite and analcite, and has been called sodalite-syenite. Its chief constit-