STUDIES FOR STUDENTS. 535 



most admirable means of working out glacial history by deter- 

 mininar the intervals between the successive moraines and the 

 topographic conditions under which they were formed. As the 

 gradient of the streams that formed these valley tracts may be 

 estimated, they afford valuable criteria for determining the 

 altitude and the attitude of the land at the time of their 

 formation. 



(2). Loess sheets. — The above valley tracts grade from the 

 coarsest gravel through sand to the finest silt. So long as they 

 are confined definitely to the valleys, a sub-classification of them 

 on the basis of coarseness or fineness of material would be rathei 

 structural than genetic though carrying a genetic significance. 

 But when the waters spread widely beyond the immediate vicin- 

 ity of the valley and the material took on a peculiar and dis- 

 tinctive assortment, there seems to be sufficient ground for 

 recognizing a second genetic class. The great tracts of fluvial 

 loess (not all varieties of loess) are placed here. These are 

 valley phenomena, in part. They have for their axes the great 

 valleys of the region and their thickest deposits are along the 

 valleys. They, however, spread widely over the adjacent 

 country so that conjointly they mantle the whole region. They 

 also coalesce with the great fringing sheets of loess that are 

 classed above as outwash deposits. The class graduates into 

 typical valley deposits on one side, into typical fringing deposits 

 on another, and, apparently, into wind deposits of loess on a 

 third side. 



2. By fringing lakes. 



This class obviously embraces deposits of suspended material 

 brought out from the ice into bordering lakes by glacial streams 

 and spread over their bottoms, being generally of the clayey 

 type, sometimes bearing lacustrine fossils, sometimes not ; some- 

 times commingled with stony material dropped by floating ice 

 derived from the edge of the glacier, sometimes not, at least not 

 in notable quantities, and always more or less commingled with 

 wash from the adjacent land not covered by ice. Theoretically, 

 the type is characterized by the peculiar border of the deposit 



