Geological Papers. 99 



often cut. The most notable class of this type head in ter- 

 minal moraines. Hence this drift affords valuable criteria 

 for determining the altitude at the time of the formation. 



Other classes of valley drift are the outwash and overwash 

 apron deposits already described. Loess is also classed under 

 this heading by a majority of the writers on the subject. 



Loess. — To quote Geikie : "This name was given to the de- 

 posit in Germany, where the deposit was first studied. It is 

 usually a yellowish, homogeneous clay or loam, unstratified, 

 and presenting a singular uniformity of composition and struc- 

 ture. When carefully examined, its quartz-grains are found 

 to be remarkably angular, and its mica plates, instead of be- 

 ing deposited horizontally, as they are by water, occur de- 

 pressedly in every possible position and with no definite or- 

 der. The chief constituent of loess is always hydrate silicate 

 of alumina, in which the scattered grains of quartz and flakes 

 of mica are distributed. . . . Here and there the lime is 

 segregated into concretionary forms by the action of in- 

 filtrating water. Though a firm, unstratified mass, it is trav- 

 ersed by innumerable tubes, formed by the descent of root- 

 lets, and mostly incrusted with carbonate of lime. These have 

 generally a vertical position and ramify downwards. These 

 pipe-like lime concretions have a tendency to give a vertical 

 jointing to the mass. The loess contains organic remains, 

 chiefly land shells, sometimes in immense numbers, also bones 

 of various herbiverous and carniverous animals, which are 

 either identical with or closely related to species that abound 

 on steppes and grassy plains to-day. Fresh-water shells are 

 usually rare, and marine forms do not occur. Loess is found 

 at elevations ranging from 5000 to 8000 feet above the sea 

 in China, in which country it ranges in thickness from 500 

 to 2000 feet. It also occurs in most of the glacial regions of 

 the earth. Various theories have been proposed in explana- 

 tion of this singular deposit. By some it has been referred to 

 the operation of the sea; by others to the work of lakes and 

 rivers. But its wide extent, its independence of altitude or 

 contours of the ground, its uniform and unstratified character, 

 the unworn condition of its compact particles, and the nature 

 of its organic remains, show that it cannot be assigned to the 

 action of large bodies of water. They, in fact, seem to be 

 due, in the main, to the long-continued drifting and deporting 

 of fine dust by wind over areas more or less covered with 



