RELATIONS BETWEEN CAMBRIAN AND BELT TERRANE 213 



Explanation of apparent conformity. — The reason for the apparent con- 

 formity in strike and dip between the two groups appears to be as fol- 

 lows : In pre-Cambrian time the Belt rocks were elevated a little above 

 the sea, and at the same time were slightly folded, so as to form low 

 ridges. One of these ridges is now the base of the Spokane hills, where 

 the Helena limestone and the upper portion of the Spokane shales Avere 

 removed by erosion in pre-Cambrian or early Cambrian time. Usually 

 there is very little, if any, trace of this pre-Cambrian erosion contained 

 in the basal sandstones of the Cambrian. On Indian creek, however, 

 west of Townsend, which is on the strike of the Spokane Hills uplift, the 

 basal bed of the Cambrian is made up almost entirely of fragments of 

 the subjacent Spokane shales. Fragments of these shales were also ob- 

 served in tlie sandstones of the Cambrian in the Little Belt mountains 

 near ^\^olsey postoffice. These illustrations are exceptional, the base of 

 the Cambrian sandstone being formed usuall}^ of a clean sand, such as 

 might be deposited where the sea was transgressing on the land. 



The gentle quaquaversal uplift of the Belt rocks gave them a slight out- 

 ward dip toward the advancing Cambrian sea, so that the sediments laid 

 down on the Belt rocks were almost concentrically conformable to them. 

 Subsequent orographic movements have elevated the Belt rocks into 

 mountain ridges and have tipped back and in many instances folded the 

 superjacent Cambriaii rocks, but the original concentric conformit}' be- 

 tween the beds of the two series remains wherever the lines of outcrop 

 are at right angles to the ])]ane of erosion of the Cambrian sea which cut 

 across the Belt rocks toward the center of u])lift. 



Extent of ten conformity. — 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 Cam- 

 brian. The greatest example of erosion is in the Spokane hills, where the 

 Helena limestone, with its superjacent Marsh and sul)jacent Empire 

 shales, has been removed (figure 2, page 211). In other localities the 

 red Spokane shales have been very largely removed, but some of these 

 are so far from the S])okane Hills section that it may be urged that they 

 were not originally deposited in au}^ greater thickness than is shown in 

 the sections. The unconformity now known ])roves that in late Algon- 

 kian time an orographic movement raised the indurated sediments of 

 the Belt terrane above sealevel, 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 



