tae og 1 oh eh 
aR 
R. Pumpelly—Secular Rock-disintegration. 2 137 
hardness of the material would play no causative part. The 
prominent features, such as ridges and hills, would consist of 
the rocks that are most resistant to carbonic acid, as for instance, 
the soft clay slates and mica schists, as wel] as the hard quartz- 
ytes and sandstones. The action of frost does 108 concern us 
here, for it is comparatively a surface phenomenon. 
The depressions would represent rocks more or less easily 
acted upon by carbonic acid, water and free oxygen, and to a 
greater or less extent, all rocks carrying a feldspar in abundance. 
e need not expect to find here inequalities of surface of the 
types produced by erosion. The depressions, instead of being 
valley systems caused by erosion by gravitating material, aré 
necessarily, to a great extent, closed basins produced by the 
downward growth of the rock decay. Deep and shallow basins 
without outlets may be formed by the decay of the rock occu- 
pying the meshes of a network of less affected dykes or veins ; 
but generally they owe their origin to the unequal distribution 
of the sources of dissolving reagents on the surface above, for 
These conditions are all presented i in the most striking manner 
on the plateau of Central Asia. 
*The importance of this decay in a ld under 8 oipegce: from erosion seems to 
have wholly escaped the attention of geologists as a factor of general significance. 
ven the more remarkable local — are Seach mentioned and only one or 
two pare found references in text 
r. Darwin states that the atieratia , Waveland s in Brazil have all been surprised 
at ths depth to which the A est — other eee! rocks as well as the talcose 
Slates in the interior hav “a 
fice ear Rio every aaa exce 4 “the rtz has been completely softened, in 
me places to depths little less than tae s Himdred feet.—Darwin, Geological 
Observations Part IIT, p. 143. London, 1851. : 
ofessor J. D. D. Whitney explains the accumulation, sometimes thirty feet thick 
of unstratified red clay, chert and gelens t in the Neb tas nsin lead region, by ing Ba 
y the gra 
n a 
scale among the dolomites of cone Missouri.— ogical survey of Missouri ; 
ecording to Dr. Benza, the Wélighowy Hills, occupying an area of about 700 
Square miles in southern Hindostan are of granitic and egy rock and are 
Fattoaratedt often to a depth of Serie: feet.—Leonhard u. Brown, Neues Jahrb. 
ir Mi 
fi Geol. u. Petrofact., 1838, p. 713 
memoirs of the eal urvey of India record the prevalence of this 
deep-seated decay in the crystalline rocks of that part of the world. 
Dr. Hunt, who considers this decay to hay en place in Archean 
tim affirming its prevalence pths in the southeastern States, sees 1 
it the source of the exte deposits of hydrated iron ores that occur along the 
base of the Bl Ridge. He s its removal in the 
ue ascribe 
to the action of long-continued erosion and the final washing away by wate 
i ise the Glacial epoch.—Proceedings Bost. Soe. of Nat. Hist., vol. xvi, " Oct 
18 
