GEOLOGY — WILLIS. 217 



The head of an alluvial cone may lie high upon the mountain side, 

 and in some instances such a one has been chosen as the site of a mon- 

 umental tomb. That of Ch'i-yu-fbn, 20 miles northwest of Peking, 

 is a striking instance ; the Ming Tombs are in part similarly situated. 



Constitution of the plain. — It will be noted that the plain of Peking 

 is the surface of that earthy, sandy, and gravelly material which 

 fills the embayment in the mountains. The constitution and arrange- 

 ment of this material is of fundamental importance in the question 

 of underground waters which may be reached by a deep well, and it 

 has been made the object of such observation as is possible without 

 boring. At Peking the surface, so far as can be seen, is composed 

 of fine silt and sand. It has been worked over in the gradual growth 

 and decay of the city or cities during ages, until its original condition 

 and arrangement are lost. It is stated on good authority that founda- 

 tions of old houses are found 25 feet below the present level. Three 

 miles west of the west gate of the Chinese city, on the road' to 

 IvU-k'ou-kiau, very coarse gravel, mingled with fine sand, begins 

 and is continuous thence to the Hun. This gravel is plainly the 

 material distributed by the Hun in its latest excursions over the 

 plain, together with dune sand and dust blown from other areas. 



The valley of the Hun was examined as far up as San-kia-tien. 

 It consists of an inner canyon of unknown depth, which is nearly 

 filled with sand and gravel, and of an outer valley of moderate width. 



In the northwestern part of the Bay of Peking the base of the 

 mountains was skirted for 8 miles, from Chai-k'ou to Yang-fang. 

 Near the hills connected alluvial cones constitute a continuous zone 

 of coarse gravel, which gives place, a mile or more further out, to 

 a belt of sand, on which are situated the towns named Ch'ien-sha- 

 kien and Hou-sha-kien, i.e., Front Sand Town and Back Sand Town. 



From Yang-fang to Chang-p'ing-chou, a course of 7 miles north- 

 east across the several streams which form the Sha, or Sandy River, 

 the western branch of the Pei, the plain is nearly level and is com- 

 posed of sandy loam. This loam packs hard, cracks in drying, and 

 is separated by the wind into two parts : fine dust which rises in 

 clouds in the air and sand which is blown along the ground. 



From Chang-p'ing-chou east to T'ang-shan Hot Springs, 10 miles 

 along the mountain base, coarse wash from the hills lies in a belt 

 north of a zone of sand and sandy loam, in relations like those noted 

 along the western side of the bay. 



From T'ang-shan south to Peking, iS miles down the central line 

 of the bay, the prevailing material is sandy loam like that already 

 described. South of each of the streams flowing east is, however, 



