284 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



unconformably resting on the pre-Cambrian Bow River sedimentary 

 series. His criticism of this section is that I did not graphically illus- 

 trate the unconformity, and the same comment is made on the dia- 

 grammatic section near Helena, Montana.^ 



Of the Bow River Valley unconformity I wrote : ^ 



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 for- 

 mations, Fairview of the Cambrian and Hector of the Algonkian, dip away 

 at about the same angle, but, wlten we apply the test of the varying thickness 

 of the basal Cambrian 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 Valley 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 irregularities of the pre-Cambrian surface. 



The unconformity is well shown at Fort Mountain, where the basal Cam- 

 brian 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 contain 

 rounded and angular fragments of the subjacent pre-Cambrian formations 

 (fig. I, pi. 46). The Cambrian sea was evidently transgressing across the 

 dark, siliceous shales of the pre-Cambrian land and reducing them to rolled 

 pebbles, angular fragments, and mud. The mud gave origin to small lentiles 

 of shale similar in character to the shale below the unconformity, while lentiles 

 of sandstone of greenish tint indicate that fine material was being derived 

 from still older pre-Cambrian formations than the shale 



Of greater importance is the evidence that the sediments of the two periods 

 were deposited under different physical conditions. The Cambrian sandstones 

 are composed of clean, well-washed grains, and the Cambrian calcareous and 

 argillaceous shales were deposited as muds off shore along with the remains of 

 an abundant marine life. The Hector shales of the pre-Cambrian are 

 siliceous and without traces of life ; the sandstones are impure and dirty, with 

 the quartz grains a dead milky white, or glassy and iron stained. The sedi- 

 ments forming them were evidently deposited in relatively quiet muddy 

 waters, and I think in fresh or brackish waters. 



It should be noted that in the Bow River \'alley section the Lozvcr 

 Cambrian rests on the pre-Cambrian, while in central Montana, 

 about the Belt IMountains. it is the Middle Cambrian that is at the 

 base of the Paleozoic series. 



^ Rothpletz, p. 22. 



^ Pre-Cambrian Rocks of the Bow River Valley, Alberta, Canada. Smith- 

 sonian Misc. Coll., Vol. 53, IQTO, pp. 426-427. 



