96 ANNALS NEW YORK ACADEMY OF SCIENCES 



quartzites and shales, Walcott was led to place the whole 12,000 feet of 

 strata in the Lower Cambrian. The Fortieth Parallel geologists, reason- 

 ing that the granite at the base was pre-Cambrian in age and separated 

 from the quartzite series by a great unconformity, also assigned it to the 

 Cambrian period. It is significant that the description given by King of 

 the upper part of the section includes sheets of conglomerate, which, how- 

 ever, Walcott does not mention. These occur in a succession of coarse 

 sandstones, the individual pebbles being small, usually less than half an 

 inch in diameter. Blackwelder^*^ has called attention to the strong litho- 

 logical resemblance of these pebbles to the bright colored quartzites far- 

 ther down in the series. He has also pointed out the fact that the section 

 which is here 12,000 feet thick is much thinner to the north and that it is 

 subject to rapid variations of thickness within short distances. These 

 facts are taken to suggest the existence of an unconformity within the 

 quartzitic series. At a horizon roughly estimated to be 1500 feet below 

 the top of the quartzit-e in Big Cottonwood, Blackwelder reports the ex- 

 istence of a well-marked basal conglomerate, which he represents as lying 

 upon the truncated edges of the lower members, showing, however, little 

 angular discordance between the two sets of beds. This old erosion sur- 

 face is taken as the base of the Lower Cambrian, marking the separation 

 of the Cambrian from the Algonkian. 



At the head of South Fork, near the Eexall mine, the writer found a 

 heavy conglomerate composed of large, well-rounded quartzite and gneiss 

 bowlders lying upon a very black rock of strange characteristics, the de- 

 scription of which will be given later. Overlying the conglomerate are 

 700 feet of well-bedded white quartzite, showdng several slieets of fine 

 conglomeratic material. Above this quartzite is a shale 125 feet thick, 

 and superjacent to this comes the lowest limestone series. Tracing the 

 conglomerate northward, the underlying black formation gradually thins 

 out and the conglomerate comes to rest on the next lower bed of white 

 quartzite. Passing west of Kesslers Peak, this contact travels down the 

 east face of Mineral Fork and crosses Big Cottonwood Canyon, where 

 Blackwelder saw it, a short distance below the Maxfield mine. Maintain- 

 ing a fairly constant distance below the top of the series, the contact rises 

 rapidly on the north wall of Big Cottonwood Canyon and crosses the 

 divide into Neif^s Canyon just south of the head of that basin. Cui^ing 

 gradually to the west, it crosses the crest of the range near the head of 

 Tolcats Canyon and descends rapidly to the base of the mountain in Salt 

 Lake Valley. From the starting place at the head of South Fork, it may 



i<*E. Blackwelder: "New Light on Geology of Wasatch Mountains, Utah." Bull. 

 G. S. A., Vol. 21, p. 520. 1910. 



