628 W. H. SHERZER RECOGNITION OF TYPES OP SAND GRAINS 



medmm sand, .50 to .25 millimeter; fine sand, .25 to .10 millimeter; very 

 fine sand, .10 to .05 millimeter; silt, .05 to .01 millimeter; fine silt, .010 

 to .005 millimeter; clay, .005 to .0001 millimeter. 



Glacial Sand Type ^i 



When freshly assorted and deposited by the glacial drainage streams 

 the sand resulting from the mechanical crushing of crystalline rocks by 

 glaciers is sharp, angular, fresh, and bright under the microscope, show- 

 ing but little evidence of weathering or of wear (see figure 1, plate 43). 

 The quartz grains are strongly vitreous in a good light, show the charac- 

 teristic conchoidal fracture, have sharp edges and keen points. The 

 cleavable minerals exhibit fresh looking cleavage surfaces, seams, and 

 edges, and give but traces of internal decomposition due to incipient 

 weathering. There is generally much variety of mineral represented, 

 depending, of course, on the nature and composition of the rocks serv- 

 ing as the parent beds. The sand collected from the banl^s of glacial 

 streams or from their deserted beds is generally poorly assorted, leading 

 to considerable variation in the size of the constituent particles. Owing 

 to their size and the inability of the glacier to hold them firmly, faceted 

 and glaciated sand grains would scarcely be expected; still an occasional 

 larger grain may be found showing such facets and scratches, very sug- 

 gestive of those carried by boulders from the till.^ An occasional well 

 rounded granule may be met with, as seen in figure 1, plate 43, which 

 may belong to another phase of granular development, water-borne or 

 wind-blown, or the form may have been originally present in the parent 

 bed. Sand washed from the pleistocene till deposits exhibits the same 

 general characteristics above enumerated, but with a considerably larger 

 proportion of roughly or completely rounded granules (see figure 2, plate 

 43). These give evidence of abrasion and wear after their formation by 

 the glacier, the sharp corners and edges showing some rounding and 

 yielding a subangular type of granule. Weathered grains are more 

 numerous than in the sand from modern glaciers, and they exhibit signs 

 of external corrosion rather more pronounced in the earlier than the 

 later till deposits.'^ Washed from these till beds, assorted and still fur- 

 ther rounded by water action, we have the source, undoubtedly, of much 

 of the sand of glaciated and adjacent regions. Such sand subjected to 



« Note the remark of Mackie in Edinburgh Geological Society, vol. vil, 1896, p. 151. 



' Sorby : On the structure and origin of non-calcareous stratified rocks. Proceedings 

 of the Geological Society of London, vol. xxxvi, 1880, p. 59. 



Evans : Felspars in sedimentary rocks as indices of climate. Transactions of the 

 Edinburgh Geological Society, vol. vii, 1898, pp. 445 and 459. 



