28 The American Geolo(/!xt. juiy, isos 
stances, for so far it has gone no farther than mere disintegra- 
tion. The production of a residual soil is a much slower process. 
The case of the Iloosac mountains mentioned above, may be 
one in which a preglacially decayed rock has remained unre- 
moved through the entire period of ice erosion. Whether this 
be so or not we have in the driftless area of Wisconsin, a very 
remarkable instance of a pre or interglacial residual soil in a 
region which was surrounded by ice, though itself untouched by 
the ice.* In this area there is a widespread clay, quite unlike 
drift clavs in that it is never stratified, and contains no angular 
fragments. It is an exceedingly tenacious clay, retaining mois- 
ture, and when it dries forming a "joint clay." The grains of 
silica are somewhat dissolved and etched by solution and weath- 
ering, the alkaline bases are entirely decomposed and the more 
soluble residues removed, while only soluble substances remain. 
From the decayed limestone and dolomite strata, the lime and 
magnesia, together with such alkalies as may have been present, 
are dissolved awa}', the mechanically included foreign materials 
alone remaining. The average depth of this residuary material 
over the driftless area is 7.08 feet. 
A more marked case of secular decay is found in the Atlantic 
coast states south of the glacial belt, t The mica schists of 
Pennsylvania and Maryland are often disintegrated to a depth of 
from fifteen to thirty feet, but the rock is merely disintegrated, 
not decayed, hence residual clay is not common. In Virginia 
and the Carolinas the rocks are often decayed to a depth' of one 
hundred feet. Of this region. Dr. Hunt saysj that the rocks 
"are often covered to a depth of a hundred feet or more, b}' the 
undisturbed products of their own decomposition, the protoxide 
bases having been removed by solution from the feldspars, the 
hornblende and the whole rock, with the exception of the quartz- 
ose layers, reduced to a clayey mass, still, however, showing in- 
clined planes of stratification." 
Other writers have described the same region. Russell? men- 
tions a dolerite dike in the Triassic sandstone of North Carolina, 
*Chamberlain and Salisbury. The Driftless Area, Sixth Ann. Kept. 
U. S. Geol. Survey, pp. 240-358. 
fRussell. Subaerial Decay of Kocks, etc. Bull. No. 52, U. S. Geol, 
Survey, 1889. 
^Chemical and Geological Essays, 1871-78, pp. 187, 250. 
§Bull. 52, U. S. Geol. Survey, p. 12. 
