320 STUDIES FOR STUDENTS 



tary cycles, each of which contains a strongly marked formation of 

 mud-cracked red shales, the shales alternating with sandy strata, and 

 both judged to have been deposited on the flood-plains of rivers whose 

 deltas had gained over the subsidence, finally filling up and displacing 

 the shallow epicontinental sea. The cycle is thus seen, not only to 

 pass from arenaceous and argillaceous to calcareous formations and 

 back again, but to pass from land to sea, and back again to land, the 

 latter transition being marked, not by a plane of unconformity, but 

 by subaerial river aggradation. Ancient land surfaces are not to be 

 recognized alone by the work of erosion, but may be surfaces of sedi- 

 mentation, and resemble in this respect the work more usually done 

 within the domain of the sea. 



In the Grand Canyon region it is concluded that at least a large 

 part of the Unkar terrane, with its 6,830 feet of strata, was built up 

 by subaerial aggradation as the delta plain of a large river exposed to a 

 climate characterized by frequent seasons of desiccation alternating with 

 seasons of flood. Thus the detailed examination of these late Pro- 

 terozoic terranes confirms by largely independent reasoning the general 

 expectation arrived at in the earlier part: notable amounts of con- 

 tinental deposits being here found collected in geosynclinal basins 

 formed within the continental platforms during this area of wdde land 

 extension. The agreement of conclusions from the geographic and 

 stratigraphic lines of approach is felt to strengthen the degree of 

 probability that in these instances the indications as to origin have 

 been correctly interpreted. 



INTRODUCTION 



As pointed out by Walther in 1893, ' although all geologists are famil- 

 iar with the occasional extensive deposition of land-waste upon the land 

 as among the results of geological activities at the present time, yet 

 the prevalence of erosion on the land and of sedimentation beneath 

 the sea has governed the interpretation of nearly all ancient sedimen- 

 tary deposits. It has ordinarily been accepted as a geological prin- 

 ciple that ancient continental surfaces are only determined by uncon- 

 formities, while, on the other hand, all sediments, unless obviously 

 deposited by fresh waters as proved by their organic contents, are 



I Einleitunghi die Geologic, pp. 719, 720. 



