110 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 
eS 
ates 
Fic. 32.— Train of residual quartz fragments derived from a vein during the 
weathering and ablation of the crystalline schists. Near Curityba, Paran4. 
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 which 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 
1 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. 
