REVIEWS. 635 



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 thev 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, con- 

 stitute 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 feld- 

 spar and hornblende, and sometimes quartz and mica. The porphy- 

 ritical 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 " Anden- 

 diorit," 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, allotrio- 

 morphic grains of an isotropic substance, probably sodalite. The rock 

 contains 5.50 per cent, of K2O, and 4.14 per cent, of Na20. The augite- 

 syenite from the Highwood mountains is coarsely granular, and con- 

 tains S-66 per cent, of K2O and 7.88 per cent, of Na20. 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 High- 

 wood mountain is characterized by a noticeable percentage of sodalite 

 and analcite, and has been called sodalite-syenite. Its chief constit- 



