408 THE ICE AGE IN NORTH AMERICA. 



a cliff is undermined, the loess breaks off in immense vertical 

 plates leaving again a perpendicular wall. 



It is divided into beds varying in thickness from one foot 

 to two or three hundred, which thin out to nothing at the bor- 

 ders and are separated by parting planes. These planes are 

 marked by angular debris near the mountains, and by elon- 

 gated upright calcareous concretions elsewhere. 



This remarkable combination of softness with great strength 

 and stability of exposed surfaces is of inestimable value in a 

 woodless country. In Asia, thousands of villages are excavated 

 in a most systematic manner at the base of cliffs of loess. 

 Doors and windows pierced through the natural front give 

 light and air to suites of rooms which are separated by natural 

 walls, and plastered with a cement made from the loess concre- 

 tions. These are the comfortable dwellings of many millions 

 of Chinese farmers, and correspond to the ruder dug-outs of 

 Nebraska. 



To the same qualities is due the fact that the loess districts 

 of China are exceedingly fertile plains, in each of which a rap- 

 idly progressing erosion has excavated the most labyrinthine 

 valley systems, in winch all the members, down to the smallest 

 tributaries, are sunk with vertical walls to depths of from one 

 hundred to several hundred feet. Even the wagon-roads be- 

 come in time depressed to a depth of fifty feet and more by the 

 removal of the dust by wind. 



There is one more peculiarity of the loess : it not only is 

 wholly unstratified, but it contains the remains of only land- 

 animals and especially of land-snails. Alexander Braun ex- 

 amined 211,968 specimens of shells from the loess of the Rhine 

 between Basle and Bonn, and found that all were land-snails 

 except only thirty-three individuals, consisting of Limncea, 

 Planorbis, and Vitrina, which came from three isolated points 

 in the valley of the Rhine and Neckar. 



Baron Richthofen, after prolonged study of the loess of 

 China, propounded the theory that it was a wind-deposit, 

 consisting of material which had resulted from the slow dis- 

 integration of the rocks in the arid regions of central Asia, 

 from which the westerly winds have been continually blow- 



