GEOLOGICAL IMPORTANCE OF SEDIMENTATION 451 



are encountered widely separated in time, deforming older basins of 

 sediments, and resulting in erosions and unconformities. It is not 

 to be implied, therefore, that such formations as the Belt of Montana 

 and the Grand Canyon terranes, because they lie immediately below 

 the Middle Cambrian, were necessarily deposited in an immediately 

 Pre- Cambrian period. 



From the thickness of their detrital accumulations situated in the 

 interior of the continent, they probably belong to interior basins of 

 subsidence within rather wide land masses. That these basins were, 

 at least during a part of the time, connected with the open sea is 

 shown by limestone formations, thousands of feet in thickness, which 

 they contain. 



In the Lewis and Livingston Ranges Willis has measured a thick- 

 ness of the Algonkian of about 10,000 feet, without either the upper 

 or lower limits being visible. Of this from 4,500 to 5,300 feet are 

 argillites and quartzites, while 5,400 feet are limestones, divided into 

 two great formations.' 



Near Helena, in west-central Montana, Walcott has estimated the 

 total thickness of the Belt terrane at 12,000 feet, of which 7,600 feet 

 consist of argillites and quartzites, the remainder, as before, being 

 divided into two great limestone formations.^ This region is possibly 

 not far from the limits of the original basin during much of the time 

 of deposition, since the Belt formations disappear from between the 

 older gneisses and the younger Cambrian some 60 miles to the south. 



In the Pre- Cambrian Grand Canyon series, exposed in northwest 

 Arizona, Walcott gives the Unkar and Chuar terranes a combined 

 thickness of 11,950 feet, of which about 400 to 500 feet are limestones, 

 and about 1,000 feet lavas. The remainder are sandstones and 

 argillites. 3 



In view of these great thicknesses of sandstones and argilHtes 

 accumulated in interior basins of Montana and Arizona at a time 

 of at least considerable continental extension, the hypothesis of a 

 subaerial origin by river aggradation may well be held in mind as 

 a possibility for certain formations, as well as the more common 



1 Bulletin of the Geological Society of America, Vol. XIII (1902), pp. 316, 317. 



2 Ibid., Vol. X (1898), p. 204. 



3 Fourteenth Annual Report, U. S. Geological Survey, Part II, pp. 508-12. 



