204 RESEARCH IN CHINA. 
are approached, and close to them rises to as much as 25 to 30 feet in a mile, 
4.5 to 5.5 meters in akilometer. The plain consists of the Huang-t’u forma- 
tion, of which loess is the predominant constituent. Elsewhere sorted 
out by wind, the loess has reached its present resting-place as the alluvium 
of the Huang-ho and other streams, which traverse the western loess- 
covered districts. It is associated with quantities of coarser sand, and 
the two are vigorously re-sorted by the wind in all favorable localities. 
The streams, in times of torrent, redistribute the material over wide flats, 
from which, in the autumn and winter season, the winds carry away the 
loess in great clouds, and over which they distribute the sand in dunes. 
We thus find more or less extensive stretches of sandy soil or pure sand 
adjacent to the rivers, and wide areas of loess in the interspaces. The 
loess is universally cultivated; and so far as possible the sandy soils are 
improved, where they lie within reach of the river floods, by the construc- 
tion of walls, which check the muddy waters and cause a deposit of the 
rich loess soil within the inclosure. The aspect of the plain is so modified 
by long-continued cultivation and artificial constructions that its natural 
features are subordinated. It is a vast alluvial flat, constantly swept by 
winds in the seasons when it is bare of vegetation and consequently covered 
by the sorted products of wind action, which are taken up in the season 
of torrential rains and redistributed by water, to be again sorted and 
redistributed by wind. 
The margin of the foothills—We skirted the edge of the foothills from 
Wan-hién to beyond T’ang-hién, and entered among them near Si-ta-yang, 
making excursions into them northwest of Wan-hién and from ‘T’ang-hién 
as far as Nan-t’ang-mei and Mi-chéng. At the base of the more continuous 
heights is a fringe of detached hills, which rise from the plain as islands rise 
from the sea. They are, indeed, rocky summits, completely surrounded by 
the deposits of the plain. They attain altitudes of 100 feet to 1,200 feet, 
30 to 350 meters, and many of them are extremely rugged in aspect. The 
summits vary from little points of rock just sufficient to afford an elevated 
position for a pagoda, such as that south of Chuang-li, to masses of con- 
siderable extent, such as that which forms the range north of T’ang-hién. 
Approaching them, one ascends over the gradually increasing slope of the 
loess, which is blown up onto them from the plains and passes frequently 
by a tangent curve to a slope of hard rock. Elsewhere, at points where the 
action of wind or water scours away the Huang-t’u, the rocks rise steeply. 
In ground plan the hills are extremely irregular, as they are in profile. 
They are composed indifferently of gneiss or limestone, and exhibit no 
special arrangement with reference to the structure of the subterrane, 
except such as is determined by the survival of heights of hard rock and 
the excavation of valleys on softer ones. They are thus features of mature 
