136 R. Pumpelly—Secular Rock-disintegration. 
exposed enjoyed a peripheral climate with a protecting vegeta: 
tion and abundant generation of carbonic acid, the feldspathic 
rocks have been profoundly affected; granite and_ gneisses 
being decomposed often to the depth of several hundred feet. 
n regions underlaid by impure limestones of great thickness, 
a long continued existence as dry land results in the removal 
of the lime carbonate and the formation of a residuary accumt- 
lation of the insoluble impurities. Thus in Missouri, in the 
zark Mountains, the secular dissolving away of the limestones 
which contained from two per cent to nine per cent of insolu- 
ble silica and clay has left such residuary deposits 20 to 120 
feet thick. 
The decay of a rock mass progresses from the joints and 
cracks, by which the mass is divided into polygonal blocks, and 
the rate of this progress varies, in different rocks, with | 
nature of the mineral constituents and the physical condition 
of structure, texture, mineral combination, ete. 
the greatest shrinkage olume, leaving masses of sandy clay 
with chert separating the broken up representatives of such 
layers of shales and sandstones as were interstratified with them. 
Calcareous sandstones and shales also leave sands and clays. 
ecay may extend to great depths before it reaches the 
cores of the larger polygonal blocks. Where this is the case 
the disintegrated mass consists of the rounded cores of the 
blocks surrounded by the decomposition product of the rest 
