PHYSIOGRAPHY OF NORTHWESTERN CHINA. 245 
effectiveness of the wind to special climatic conditions and thus laid the 
foundation of an intelligent understanding of this extraordinary deposit. 
It is to be regretted that in his day the criteria of physiographic interpre- 
tation had not been developed, else his keen eye had clearly discerned 
the history recorded in the mountain forms, and we should have had from 
him a correct interpretation of the later geologic history of Asia. 
Origin of materials of the Huang-t’u.—The distribution of the Huang- 
t’u formation has been described in the preceding chapter, and the reader 
is referred to that description for those details of occurrence which we 
observed. In this place we have to present a statement of the conditions 
of accumulation of the Huang-t’u formation, as we understand them, that 
accumulation being a chapter in the physiographic history which peculiarly 
distinguishes the Hin-chéu epoch. Building upon the observations of our 
predecessors, Pumpelly and von Richthofen, we present the following as 
the best explanation which we have been able to reach of the complex 
phenomena. 
The first question is the origin of the material of which the Huang- 
t’u is composed. Its volume and its character are unusual and challenge 
explanation. Pumpelly attributes the material to secular disintegration 
of rocks during a prolonged period of climatic conditions favorable to 
vegetation and rock decay. He says:* 
The one weak point in von Richthofen’s theory is in the evident inadequacy of the 
current disintegration as a source of material. When we consider the immense area cov- 
ered by loess to depths varying from 50 to 2,000 feet, and the fact that this is only the very 
finest portion of the product of rock destruction, and again that the accumulation represents 
only a very short period of time, geologically speaking, surely we must seek a more fertile 
source of supply than is furnished by the current decomposition of the rock surface. 
It seems to me that there are two important sources: 
I. The silt brought by rivers, many of them fed by the products of glacial attrition 
flowing from the mountains into the central region. Where the streams sink away, or 
where the lakes which received them have dried up, the finer products of the erosion of 
a large territory are left to be removed in dust storms. 
II. The second and, I believe, the more important source is in the residuary products 
of a secular disintegration which we will now consider. 
In all regions where the soil is protected by a luxuriant vegetation, the greater part 
of the insoluble products of disintegration remains 7m situ. Considerable portions of the 
continents have remained above water during long geological periods. Where this has 
been the case, and where the region thus exposed enjoyed a peripheral climate with a 
protecting vegetation and abundant generation of carbonic acid, the feldspathic rocks 
have been profoundly affected; granite and gneiss being decomposed often to a depth 
of several hundred feet. 
* Relations of Secular Rock Disintegration to Loess, Glacial Drift, and Rock Basins. American 
Journal Science and Arts, vel. xv, February, 1879. 
