GEOLOGICAL IMPORTANCE OF SEDIMENTATION 2>Z3 



ing, seme 14,000 feet of river deposits were laid down in the Siwalik 

 formaticn of the Upper Tertiary in the northwest Punjab.' In 

 searching the past for similar deposits, from the nature of their origin 

 they need be looked for only upon the ancient mountain forelands, 

 and only after periods of erogenic revolution. 



BASIN DEPOSITS OF PLUVIAL CLIMATES 

 The basins of the Great Lakes may be taken as good examples of 

 down-warping in continental interiors. Formerly regarded by many 

 as largely owing their origin to ice-erosion, they are rather looked upon 

 at the present time as chiefly due to crustal warping, somewhat accen- 

 tuated and scoured clean by the Pleistocene glaciation. Their 

 recency, and the fact that no rivers laden with detritus from mountain 

 regions flow into them, cause them to be still unfilled and their basin 

 nature to be clearly apparent. It would seem that such basins are 

 not unique, but are rather constant features of the continental plat- 

 forms. In times of diminished land surfaces and lessened erosion 

 these may be connected with the oceans and exist as epicontinental 

 seas. In times of wider continental extension and increased erosion 

 they are likely, if shallow, to soon become completely filled with sedi- 

 ment, after which rivers will flow through them on their way to the 

 sea. In this event only a geological study of the region may demon- 

 strate the basin-like structure of the underlying basement. 



Perhaps the most conspicious examples of interior continental 

 basins receiving large quantities of river sediments at the present 

 time are to be found in South America. The Brazilian plateau, like an 

 island, is surrounded on all sides by a wide lowland, at no place more 

 than 650 feet above the level of the sea. The headwaters of the Para- 

 guay and the Guapore, the latter one of the southernmost tributaries 

 of the Amazon, with hardly 4 miles between them, are often covered 

 by the same floods.^ The greater portions cf the great river valleys 

 of South America are underlaid by Tertiary deposits, and the super- 

 ficial formations are of recent origin. ^ This is indicated by any good 

 map, where it is seen that the central basin of the Amazon possesses 

 braided rivers, lagoons, and unexplored distributaries connecting 



^ Geikie, Text Book of Geology, 4th Ed., p. 1297. 



2 J. Batalha-Reis, M.il\'s International Geography, pp. 865, 866. 



3 Ibid., p. 867. 



