2l8 REPORTS ON INVESTIGATIONS AND PROJECTS. 



a belt of sand, sometimes forming dunes lo feet or more in height. 

 These zones are produced by the north winds, which sweep the sand 

 from the wide, slightly hollowed channels, in which the water is but 

 a thread during much of the year, though a flood for a brief season. 



Summing up these notes, it is apparent that the Bay of Peking 

 is surrounded by a zone of coarse gravel next the mountains, within 

 which lies a belt of sand also washed from the mountains, and that 

 the great central area consists of fine sandy loam of a rather compact 

 nature. This loam, on being redistributed by wind, gives rise to 

 local sandy tracts and to dust storms. 



Origi7i of the plain. — From the preceding description it will be seen 

 that the Plain of Peking is the upper surface of a body of material 

 which fills a depression among the mountains as water fills an embay- 

 ment along the coast. It appears also that the surface as we now 

 see it is composed of wash deposited by rivers, together with local 

 formations produced by wind. To these facts we may add the con- 

 ception, concerning which geologists in general would agree, that 

 the land formerly stood higher with reference to sea level and has 

 gradually subsided. 



During this subsidence the sea and the rivers have been in con- 

 stant conflict, the sea ever seeking to spread over the .'^inking land, 

 the rivers ever bringing down sediment to build out the land. The 

 delta of the Mississippi or the Nile or the Huang-ho illustrates the 

 work of the rivers. 



The subsidence and the filling-iu have progressed gradually. 

 There has been abundant time for the work of subsidiary activities 

 on the surface of the filling, and we may thus conceive of the material 

 beneath the present plain as having been spread and redistributed 

 by streams and winds, as we now see it spread and distributed. At 

 any particular stage of the process, at any level, there was coarse 

 gravel near the hills and resting on bed-rock, there was sand along 

 its inner margin, and the central area of the plain was composed of 

 irregular stretches of sand and fine silt or clay. 



A drill-hole sunk through this deposit should pass through many 

 layers of clay, sand, and gravel of various textures and mixtures. 

 Some may be small in area and isolated ; others may be very extensive 

 and connected with strata at much higher levels. The lowest, next 

 to bed-rock, is probablj^ the most extensive, as it must underlie the 

 whole plain, the most open because it consists of the coarsest mate- 

 rials, and the one reaching to the highest levels, since it extends up 

 onto the present mountain slopes. 



Water may be found in any of the sandy layers. An artesian flow 

 is most likely to be secured from the deepest. 



