26 The American Geologist. July, isoar 
depth, and in many regions a deep residual soil has been the 
result. The best conditions for the formation of such a soil are 
a moist climate and a region of abundant vegetation which sup- 
plies to the percolating water the necessary acids with which the 
chemical alteration of minerals is advanced. It seems also that 
secular decay proceeds more rapidly in warm than cold climates, 
though of this we have as yet no fair test, since the regions in 
the north so far studied have been recently glaciated and their 
residual soils removed. Theoretically this should be a factor of 
importance, for the abundance of decaying vegetation, the amount 
of rainfall, the duration of time when, through absence of frost, 
the water can percolate into the soil from the surface, and the 
greater temperature of the water in warm countries are all in 
favor of the greater activity of rock decay in such regions. 
{h) Characteristics of Residual Soils. 
The ultimate result of secular decay is essentially a soil of un- 
iform character, whatever the nature of the rock may be. This 
is so because as the process continues chemical alteration proceeds 
to the last degree, the greater part of the soluble salts are re- 
moved, and only^ the insoluble parts remain, and these are quite 
uniform in character. Thus Cnamberlain and Salisbury* state 
that in the driftless area of Wisconsin the residual soil on the 
sandstone ditfers but little from that on the limestone except that 
it is compact on the surface and more siliceous below. 
The liumediate product of decay, however, depends largely 
upon the character of the source. According to professor Pum- 
pellyt granites, syenites, gneisses, diorites, etc., produce argil- 
laceous sand or sandy clay ; porph3'ries. basalts, trach^-tic rock?, 
etc., produce ferruginous clays; impure limestones and dolomites 
undergo the greatest shrinkage leaving masses of sandy clay, 
often with chert; and calcareous sandstones and claj's also leave 
sands and clays. 
A residuum from secular rock-disintegration consists, in its 
best development, of several zones, grading insensibly down- 
ward into the undecom posed rock. On the surface it is a clay, 
more or less siliceous, composed almost entirely of insoluble 
minerals in a very fine state of division. This zone extends as 
*Sixth Annual Rept. U. S. Geol. Survey, pp. 240-58. 
jAm. J. Sci. 1879, xvit, pp. 133-144. 
