Weed and Pirsson — Bearpaw Mountains of Montana, 199 



spars around some object, with the palisade structure around 

 the edge, that we have thought best to give a drawing of it 

 (fig. 5), which shows also the character of the rock in a dia- 

 grammatic way ; one pseudo-leucite is shown, with a number 

 of sodalites and a prism or two of augite lying in the dark 

 mottled base with the contact with the alter-ed shale. 



Fig. 5. Contact of altered shale and tinguaite. 



The shale at the contact is altered to a dense, black, tough 

 hornstone. In the section it is found composed mostly of sub- 

 angular fragments of quartz but with other fine mineral parti- 

 cles present and mostly of an indeterminate nature ; much of it 

 appears like masses of kaolin leaves, seen in an altered feld- 

 spar ; in other pieces the grain is coarser and the rock is com- 

 posed mainly of quartz grains and particles of a pleochroic 

 brown mica. As is so often the case, there appears to be some 

 alteration directly at the contact, where the minerals are not 

 as fresh as further out in the dike. In addition to the varie- 

 ties of tinguaites described, a few others were collected from 

 the local drift of the mountains. 



The result of the study of the geology and rocks of this 

 hitherto unknown mountain group, of which only a rapid 

 reconnaissance has been made, presented in this series of arti- 

 cles, shows the region to be one of peculiarly great interest to 

 the student of volcanic geology and the petrographer. We 

 may confidently expect that our future mapping and detailed 

 study of the district will afford a rich harvest of facts impor-^ 

 tant to the science of the petrology of igneous rocks. 



Washington and New Haven, April, 1 896. 



