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bulletin: museum of comparative zoology. 



a small valley there was exposed in 1908 in the road the section 

 diagrammatically shown in Fig. 32. 



A quartz vein about 2 inches (5 cm.) thick dipping E. 30° in a 

 section of decomposed schists had given rise to a sheet of quartz 

 fragments at the base of the residual structureless surface deposits 



Fig. 32. — Train of residual quartz fragxaents derived from a vein during the 

 weathering and ablation of the crystalline schists. Near Curityba, ParanS. 

 a-b, a distance of 50 feet from the outcrop of the quartz vein to the limit 

 of fragments; c-d, the supposed surface at wliich the vein outcropped. 



traceable fully 50 feet (15.2 M.) to the westward on the gently inclined 

 surface of the schists. The original aerial extension of the quartz vein 

 from the data here presented must have been at a height of 29.5 feet 

 (8.1 M.) above the present surface as may be readily determined by a 

 calculation of the right angle triangle. The two to three feet of 

 overlying structureless residual clay may or may not represent the 

 breaking down of about thirty feet (9.15 M.) of rock above the present 

 surface of the schists. In either case solution by percolating water has 

 been the chief agency in denudation. If the removal of this thickness 

 of rock went on at the average rate for such drainage areas as have 

 been studied — a rate as great as one foot in 3,000 years, the time 

 represented in this case for the lowering of the quartz fragments is 

 approximately 88,500 years, a period which takes us back according 

 to the newer ^ estimates to the close of the last glacial epoch in North 

 America. To this estimate should be added the time for the accumu- 

 lation of the overlying clays whose superposition on the quartz 



» The most recent studies of the retreat of the Wisconsin ice-sheet and the glacial 

 lakes and marine phenomena which succeeded the glacial retreat demand from 5 to 

 10 times the 10,000 years of earlier estimates. 



