Weed and Pirsson — Bearpaw Mountains, Montana. 287 



occasional lenticular intercalations of impure limestones. 

 So far as observed, the rocks have proved nearly barren of 

 fossils, but as a coal-bearing series occurring along Milk River 

 is also exposed beneath the drift along the stream courses 

 north of the mountains, fossils will probably be found upon a 

 close examination of the beds.* Older Cretaceous strata also 

 occur to the north, near Havre, where they have been identi- 

 fied by Mr. T. W. Stanton, and in the bad lands of the 

 Missouri River, where they have been studied by one of the 

 authors (W. H. W.). About the cores of granular igneous 

 rocks the sedimentary beds are generally highly metamor- 

 phosed, more especially where the eruptives are of basic 

 types. In such cases the original character is obscured, and 

 the shales and sandstones are converted into dense, hard horn- 

 stones and quartzites of those light-colored, compact and 

 flinty rocks of various shades of lavender, gray, green, and 

 of adinole-like character, which so often characterize the con- 

 tact zones of igneous intrusions. These baked rocks possess a 

 marked cubical jointing, which causes them to break into small 

 angular fragments, so that imposing exposures are seldom seen. 

 As, however, they often resist erosion better than the granular 

 rocks of the volcanic cores, the contact ring generally stands 

 in bold relief, forming ridges about the igneous centers. 

 Where dikes occur thickly clustered, as is. the case on the 

 ridge at the head of Snake Creek, the sedimentary rocks are 

 also much indurated and altered. 



From what has already been stated, it is evident that the 

 chief interest in the region lies in the igneous rocks. The 

 unusual character of the types represented make their study of 

 importance, for which reason the field occurrence and the 

 petrographic character are described somewhat in detail. 



Extrusive JtocJcs. 



The extrusive rocks are most abundant, forming the highest 

 peaks and many of the lesser summits of the region ; they 

 are the usual rocks of the foot-hills, where their richly-colored, 

 rough outcrops form crags and oddly shaped castellated 

 masses in strong contrast to the smooth and grassy slopes 

 about them. They consist of dark-colored basaltic tuffs, 

 breccias and lava flows, which are parts of the former volcanic 

 cones. The finer tufaceous varieties are often washed down 

 and form dark-colored, sandy soils, generally well grassed, 

 while the vesicular slaggy rocks and coarser breccias form 



* We have been informed by Prof. 0. C. Marsh that the remains of a Dinosaur 

 Hadrosaurus breviceps Marsh) described by him (this Jour., vol. xxxvii, p. 335, 

 1889, also ibid, vol. xxxix, p. 423, 1890), are stated by the collector to have been 

 obtained in the Bearpaw Mountains. 



