Relation of Secular Decay of Rocl:.s. — Tarr. 39 
isbury make the point that the glacial deposits of Wisconsin are 
not derived from a secularly decayed soil. They state that resid- 
uary products are almost completely robbed of alkalies, contain- 
ing chiefly quartz, undecomposable silicates and ferric oxide. 
Drift claj's, on the other hand, are calcareous, and in some of 
these claj^s there is forty per cent, of magnesian limestone. ' 'The 
drift clan's are. in short, rock flour, and not, as are the residuary- 
earths, the products of rock rot.'' 
I think that this argument is not valid, for the reason that but 
a small part of secularly decaj-ed rock consists of residual soil. 
Beneath the surface it becomes more and more like the rock from 
which it has been derived and finally passes insensibly into it. 
This last must have been the material with which the ice was work- 
ing in its last stages. Moreover, in places, perhaps over the 
greater part, the ice was then at work on the undecayed rock, 
and the churning up of this with the products of disintegration and 
secular decay might readily introduce a lai'ge element of calcare- 
ous matter. 
{(■) By Rivers. 
Residual soils can accumulate onl}- where decay is in excess of 
denudation. Regions of considerable topographic diversity- are 
generally free from accumulated soils and these soils are most lia- 
ble to form where the streams are not rapidlj' eroding. As an 
instance of this reference may be made to Mr. Russell's paper* in 
which it is stated that where erosion is rapid, in the south- 
ern Piedmont region, owing to the more rapid solution of certain 
limestones, and the consequent standing out of other rocks, no 
residual soil remains. The presence of vegetation aids greatly in 
the retention of such soils, and this protective effect often allows 
the streams to run clear even when their flow is quite rapid. 
If, on the other hand, a region bs elevated and new life be thus 
given to the streams, there is such rapid erosion that there is a tend- 
ency to strip off the accumulated soils. Such a revival or reju- 
vination of the streams seems to have taken place in China. The 
streams are yellow with silt derived from the loess, and the waters of 
the Chinese sea near the great rivers are yellow from this cause one 
hundred miles from the coast and for six hundred miles along the 
coast line. The muddiness of some of the rivers of the Mississippi 
system and of the Amazon are in large part due to the same cause. 
*Biill. No. 52, C. S. Geol. Survey, 1889. 
