SEPARATION OF CAMBRIAN FROM ALGONKIAN 523 



It seems clear, therefore, that there is an unconformity not far below 

 the top of the great quartzite-slate series, and it will probably be found to 

 have a much wider distribution than now known. It corresponds in a 

 general way with the unconformities which separate the Middle Cam- 

 brian from the Belt series of Montana and from the Grand Canyon series 

 of Arizona. 



Just below the fossiliferous Cambrian shales and limestones there 

 is a quartzite 1,000 to 1,500 feet thick. This quartzite rests now on 

 the eroded surface of the Algonkian quartzite and slate, and now on 

 the much older gneiss and schist which are generally referred to the 

 Archean. Walcott has named it the Brigham quartzite, but it may be 

 seen most clearly at places such as Ogden and Willard, rather than at 

 Brigham. The oldest fossils found in the shales are referred by Walcott 

 to the Lower Cambrian. The Brigham quartzite may therefore be as- 

 signed also to the early Cambrian^* and the quartzites and slates beneath 

 the unconformity to the Algonkian. There is, however, an alternative 

 view, advocated by Daly and others, that the oldest Cambrian faunas in 

 the Eocky Mountains are Middle Cambrian and that the thin Brigham 

 quartzite is the same in age ; that the unconformity represents a brief 

 time interval and that the great quartzite slate series is not Algonkian, 

 but simply early Cambrian. Critical studies over a wide area are a neces- 

 sary preliminary to the settlement of this question. It is to be remem- 

 bered that the unconformity seems to imply the complete removal of the 

 great quartzite series during the erosion interval and over wide areas. 

 The time involved should therefore be more than a brief interruption. 



INTERPRETATION OF THE ALGONKIAN QUARTZITE-SLATE SERIES 



For the present the thick series of quartzites and slates lying below the 

 unconformity just described will be assumed to be Algonkian, according 

 to the prevalent view. In the two best known sections — that is, in Big 

 Cottonwood Canyon and along the upper course of Ogden Eiver — this 

 system has many characteristics which harmonize with a particular inter- 

 pretation of its origin. The base of the series has apparently not l)een 

 seen any where. ^^ 



The prevailing rocks are quartzites or quartzitic sandstones variously 

 colored. Here and there in the sandstone are many thin beds of con- 



1* Walcott considers the upper part of the quartzite Middle Cambrian, north of the 

 Salt Lake district. 



15 Along the back slope of the main range from Ogden Canyon north almost to Brig- 

 ham, there is a mass of folded dark slates and schistose graywackes Avhich underlies the 

 quartzite-slate series. The details of the succession and the relations of these beds are 

 yet unknown, but will repay careful study at some later time. It is not improbable 

 that two unconformable systems may be discovered. 



