288 SMITHSONIAN MISCELLANEOUS COLLECTIONS \'0L. 64 



between the beds of the two series remains wherever the Hnes of outcrop are 

 at right angles to the plane of erosion of the Cambrian sea which cut across 

 the Belt rocks toward the center of uplift. 



Extent of unconformity — The extent of the unconformity between the Belt 

 and the Cambrian may never be ascertained, as there is no section known 

 where the sedimentation is unbroken from the Belt to the Cambrian. The 

 greatest example of erosion is in the Spokane Hills, where the Helena lime- 

 stone, with its superjacent Marsh and subjacent Empire shales, has been 

 removed (fig. 11, p. 263). In other localities the red Spokane shales have been 

 largely removed, but some of these are so far from the Spokane Hills section 

 that it may be urged that they were not originally deposited in any greater 

 thickness than is shown in the sections. The unconformity now known proves 

 that in late Algonkian time an orographic movement raised the indurated sedi- 

 ments of the Belt terrane above sea-level, that folding of the Belt rocks formed 

 ridges of considerable elevation, and that areal erosion and the Cambrian sea 

 cut away in places from 3,000 to 4,000 feet of the upper formations of the Belt 

 terrane before the sands that now form the middle Cambrian sandstones were 

 deposited. 



I think that an unconformity to the extent indicated is sufficient to explain 

 the absence of lower Cambrian rocks and fossils and to warrant our placing 

 the Belt terrane in the pre-Cambrian Algonkian system of formations. 



Epeirogenctic unconformities. — Rothpletz seeks to explain the un- 

 conformity at the base of the Flathead quartzite by considering it as 

 only the result of the filling up of a basin by sediment and the subse- 

 quent depression of the basin so that sedimentation was resumed, as 

 has been the case many times in the history of sedimentary deposition 

 on the North American Continent. In advancing this view, however, 

 he fails to consider the great unconformity in the Grand Canyon area 

 where profound faulting and displacement followed by prolonged 

 erosion took place prior to the Cambrian transgression and sedi- 

 mentation ; the equally great disturbance and erosion in the Llano 

 area of central Texas ;' and the Spokane Hills uplift east of Helena, 

 where a ridge of the Belt series rocks was eroded so as to remove 

 3,000 feet or inore before the Spokane shales were buried beneath 

 the sands of the transgressing Cambrian sea and the Big Snowy 

 Mountains unconformity (p. 274). 



In a recent publication I spoke of the post- Algonkian, pre-Cambrian 

 unconformity as follows : ^ 



CAMBRIAN BASAL UNCONFORMITY 



From the Robson Peak region of British Columbia and Alberta to Arizona 

 and southern California, a distance of over 1,000 miles (1,600 km.), clear 

 evidence of a transgressing Cambrian sea has been found in many localities, 

 proving conclusively that a general unconformity occurs here between the 



Walcott, American Jour. Sci., 3d Sen, Vol. 28, 1884, p. 432. 

 Problems of Geology, Chap. 4, Yale Univ. Press, 1914, pp. 170-171. 



