412 Annals of the Carnegie Museum 



almost perpendicular masses, while the latter forms comparatively 

 smooth slopes, whether gentle or steep, and the rock has a much more 

 regular dip. These rocks, too, usually form graceful hills and moun- 

 tains with regular slopes and wide V-shaped valleys with forested 

 ravines and wooded and grassy inclines fashioned into pleasant u])land 

 copses, woodlands, and parks, even to the tops of the highest moun- 

 tains, in great contrast with mountains where the Palaeozoic lime- 

 stones predominate. Of course in the Algonkian, where series of hard 

 quartzytes prevail, the scenery is sometimes more rugged. 



West of the Rocky Mountain divide the formation is, for the most 

 part, carved into mountains, low, of medium height, and extensively 

 covered with evergreen timber. 



With the exception of the eastern portion, where a few fragmentary 

 fossils have been found and described by Dr. C. D. Walcott, the rocks 

 appear to be almost destitute of fossils. I have searched many times for 

 any vestige of plant or animal life, but have found nothing, with the 

 possible exception of tracks in the vicinity of Missoula. 



The deposits were made, at least to a great extent, in compara- 

 tively shallow water. The slates and quartzytes are commonly ripple- 

 marked and in some places these markings are the finest I have seen. 



I do not remember noticing any place where these Algonkian beds, 

 which have just been described, are resting on Archaean gneiss. 



The Bitter Root Mountains west of the Bitter Root Valley are com- 

 posed to a great extent of granite and granitoid gneiss. I once ex- 

 amined the rocks in Bear Creek Cafion. They are different from the 

 so-called Archaean gneiss east of the main divide but I noticed that 

 part of the rock in the caiion was gneiss and schist, and that it had 

 the appearance of stratification and dipped to the east. It has been 

 considered of late origin, but some of it may be metamorphic. 



Relations of Pre-Cambrian Rocks. 



The older Pre-Cambrian rocks have not been studied sufficiently 

 here to warrant any decided opinion concerning the problems in- 

 volved. I wish, however, to present one or two questions of general 

 geological importance. First I wish to avoid confusion with regard 

 to names and will define the terms as 1 shall use them. 



I. Archaean gneisses. The various gneisses and schists usually 

 having the appearance of stratification known to underlie rocks of 

 Cambrian age and often in direct contact with them. 



