Relation of Secular Decay of Mocks. — Tarr. 35 
An important contribution to tliis subject has been made by 
Baron Richtofen,* who tries to explain loess in China by this 
process. The loess in that country and in other parts of the 
world has long been a puzzle to geologists, and various attempts 
have been made to account for its origin, though as 3'et no thor- 
oughly satisfactory general theory has been advanced. It is 
probable that loess is derived in a variety of ways, and the solu- 
tion of the problem of its origin has perhaps been dela3'ed hj 
the attempt to find a universally applicable hypothesis. 
In northern China the loess covers several hundred thousand 
square miles, often attaining a depth of two thousand feet. It 
is a calcareous loam, easily crushed to an almost impalpable pow- 
der, wholly unstratified and containing only land fossils. These 
facts seem to prove quite conclusively that it is not of marine ori- 
gin, and the fact that it is found in the loftiest passes, as high as 
8,000 feet above sea level, seems to prove that it is not lacustrine. 
It is found to contain innumerable vertical tubes which, it has 
been suggested, may be the casts of grass stems; so abundant 
are these that the rock cleaves vertically, and when, by the ero- 
sion of streams, through sapping or undermining, great blocks 
fall off, vertical cliffs remain. 
Richtofen believes that these vast deposits are wind blown in 
origin A "central area" is one in which evaporation exceeds 
precipitation. The in-blowing winds, aided by surface wash, 
transport the finely divided materials into such a region. The 
steppe vegetation characteristic of such a region would help to 
catch and hold the fine dust, and this grass, buried beneath the 
aeolian deposits might leave by their casts such tubes as are 
found in the loess. Briefly this is Richtofen's theory for the 
origin of loess in China. Pi'ofessor Pumpelly accepts this theory 
and extends it to explain the loess of Missouri, f In order 
to account for the great amount of material, which seems in 
excess of rock disintegration, he supposes that rivers charged 
with products of glacial attrition carried to the region material 
for the winds to transport. 
Professor Pumpelly calls attention to the fact that, while there 
is much secularly disintegrated material in southern Asia, the 
feldspathic rocks of northern China and central Asia are as free 
*China. 1877, i, 56-189. 
tSecular Rock Disintegration, Am. J. Sci., 1879, xvn, 132-144. 
