A. L. Hwing—FErosion of Limestone. 31 
Making this correction it still leaves 275 tons per sq. mile as the 
amount annually removed in solution. 
A closer approximation could be made by continuing obser- 
vations similar to the above through a period of time, say for 
one year. To get the amount of water carried off in a year this 
would be necessary. Yet it is evident that the amount of solids 
carried away tn solution is much more constant than the amount of 
water flowing through the stream ; as in times of freshet or exces- 
sive flow there is a larger proportion of surface water, while in 
times of diminished flow a relatively larger portion of the water 
reaches the stream through underground drainage and must 
contain more solids. Hence the above number, 275 tens per 
mile, may be: taken as a fair approximation to the amount of 
material actually removed per annum in this and similar 
regions, 
by this process would require 29,173 years, or about 9,000: 
years to remove one foot from the surface. 
1000 feet of limestone has been thus removed. A sim 
further deduction shows that accordingly Nittany valley has. 
been 1,000,000 years in process of formation. 
he limestone erosion could not begin before the latter stages. 
of the Mesozoic era, possibly not before the Cenozvie era, as 
sufficient time must have elapsed subsequent to the Carbonif- 
erous age to erode all formations of the Paleozoic era above 
the Trenton limestones. One million years seems not incon- 
Sistent with other estimates of geological time; still these caleu- 
lations respecting time are an afterthought and supplementary 
io the main point intended to be shown in this paper, viz: the 
Tate and amount of chemical erosion of limestone. 
