426 SMITHSONIAN MISCHLLANKOUS COLLECTIONS VOL. 53 



and angular pebbles (fragments) of the dark siliceous shales of the 

 subjacent Hector formation: also of the siliceous and hard greenish 

 shale that occurs from 520-640 feet below, and the reddish and 

 chocolate-colored, arenaceous shale 640 feet or more below the base 

 of the Cambrian. 



Two and one-half miles north of Fort Mountain, at the east foot 

 of Ptarmigan Peak, the basal conglomerate is only 170 feet thick, 

 w'hile on Mount Temple. 8 miles southeast of Fort Mountain, it is 

 represented by a few thin layers of fine conglomerate interbedded in 

 a massive-bedded, fine-grained sandstone. 



On the north slope of Vemiilion Pass, east of Boom Mountain. 

 II miles southeast of Mount Temple, the conglomerate occurs in 

 massive beds that form a series 200 feet and more in thickness. 



The variation in thickness of the basal Cambrian conglomerate 

 seems to indicate that the pre-Cambrian surface over which it was 

 deposited was broadly irregular. 



UNCONFORMITY BETWEEN THE CAMBRIAN AND THE PRE- 

 CAMBRIAN ROCKS 



Viewed in a restricted way. much of the pre-Cambrian surface 

 was regular and the Cambrian rocks appear to be conformable to 

 the subjacent pre-Cambrian strata. All about the sides of the valley 

 the strata of the two formations, Fairview of the Cambrian and 

 Hector of the Algonkian, dip away at about the same angle, but, 

 when we apply the test of the varying thickness of the basal Cam- 

 brian conglomerate and the difference in the character of the upper 

 beds of the Algonkian in different places, we at once become aware 

 that the pre-Cambrian surface is more or less irregular, and that 

 when the Cambrian sea transgressed over the area now included in 

 the Bow^ A'alley it found a broadly irregular surface with low hills and 

 broad level spaces covered with a deep mantle of disintegrated rock. 

 It washed out the muds and carried them away and deposited the 

 sand and pebbles of its advancing beaches' over and around the ir- 

 regularities of the pre-Cambrian surface. 



The unconformity is well shown at Fort Mountain, where the 

 basal Cambrian is formed of massive layers 4-10 feet thick, which 

 usually rest directly on the Hector shale (pre-Cambrian). In places, 

 however, .slight hollows in the shale are filled with thin layers of a 

 more or less ferruginous sandstone that was deposited by gentle 

 currents prior to the deposition of the massive conglomerate layers. 

 The lower 10-20 feet of this conglomerate contains rounded and 



