alluvial clay strongly impregnated with alkali, through which run deep caiions : 

 an especially large one occuring to the east of Hui-ning Hsien (Plate 38). 



As Hsiao-shui-tzu is approached, a system of high mountains makes its 

 appearance to the south and south-west. From these mountains thestreams bring 

 down great quantities of pebbles and boulders of limestone and crystalline rocks. 



At Hsiao-shui-tzu (or Shao-shui-tzu) itself the Yellow River is again 

 reached, at this point cutting through a thick dyke of granite to a depth of 

 200 feet (Plate 33). On this granite lies a layer of green sandstone and on 

 this again a deep deposit of loess. The loess here seems to contain a certain 

 amount of whitish clay, and becomes extremely hard under the influence of 

 the hot sun. 



The formations round Lan-chou Fu are very complex. There is a good 

 deal of felspathic rock, especially north of the river, which here flows through 

 a long valley, from two to three miles wide. 



To the south of the city, sedimentary rocks are first encountered, but 

 these give way to crystalline rocks, which rise to a height of from 10,000 to 

 11,000 feet. Westward, the mountains to all appearance are formed of igneous 

 rocks, and very complicated. Gold, silver-ore, jade and precious stones are 

 obtained in this range. 



The sedimentary strata, which occur some ten miles south of Lan-chou 

 Fu, namely at Wa-kang-ch'^ng, are coal bearing ; whilst a plentiful supply of 

 clay — derived probably by decomposition from igneous rocks — allows of 

 extensive pottery-works being carried on. 



There is a good deal of grey cr3'stalline limestone in the mountains south 

 of the city. These are more or less rounded, no very prominent peaks existing. 

 Loess occurs at an altitude of over 7000 feet. 



The next stretch of country to be discussed is that along the road taken 

 by the Expedition on its return journey from Sui-t6 Chou, in North Shensi, 

 eastward to Fen-chou Fu, in Shansi, and thence north-eastward to T'ai-yiian 

 Fu. Between Sui-te and the Yellow River very deep deposits of loess and 

 Shao-fn occur. The latter is found usually at the bottoms of the deep ravines 

 and is of a deep brick-red colour. In places the ravine-bottoms are formed of 

 the usual sedimentary strata (Shensi formation) with marked horizontal 

 bedding. Within five miles of the Yellow River the loess gets very shallow, 

 and in many places the sandstone substrata are exposed in the form of rounded 

 hills with precipitous ravines. The depth (500 to 600 feet), to which the river 

 has cut through the sedimentary formation, is much greater here than at the 

 spot further north where the Expedition crossed on its journey westward. 



126 



