630 W. H. SHERZER — RECOGNITION OF TYPES OF SAND GRAINS 



and more detached crystal fragments than the glacio-volcanic subtype 

 and a complete absence of striated facets. Sand washed from the mud 

 flow that overwhelmed Herculaneum shows considerable rounding, which 

 might plausibly be ascribed to the mutual abrasion of particles during 

 the flow of a few kilometers. It seems to sustain about the same relation 

 to ordinary volcanic sand that sand washed from the till sustains to 

 freshly formed glacial material. An examination of the sand, however, 

 which fell upon Pompeii directly from the atmosphere during the same 

 eruption shows about the same degree of rounding, and suggests that in 

 both cases the cause was the same. 



Eesidual Sand Type 



Sand of this class results from the complicated set of agencies compre- 

 hended under the general term of ^Sveathering," acting on rocks of the 

 plutonic or volcanic type. Through unequal expansion and contraction 

 due to temperature changes, expansion due to ice formation in the crev- 

 ices, or hydration of constituent minerals in the body of tKe rock, the 

 surface of the bed is disrupted and disintegrates into a gravelly sand. 

 This action is hastened by various chemical changes and solution, and if 

 long enough continued results in the formation of a rusted soil. The 

 conditions under which such changes occur have been fully discussed^; 

 our chief interest here is in the resultant tj^e of sand grain. From the 

 method of formation residual sand must be poorly assorted, what assort- 

 ment there is being due to the removal near the surface of the finer 

 material by wind or rain wash and the fact that the texture becomes 

 gradually coarser as one descends toward the parent bed. This means 

 that the residual deposit is better assorted than the glacial and more 

 poorly than the volcanic. Under the microscope the granules appear 

 decidedly angular, with no evidence of secondary rounding (see figures 

 5 and 6, plate 43). Isolated crystals and crystal fragments are in evi- 

 dence, indicating a tendency of the constituent minerals to drop apart, 



» Hunt : The decay of rocks geologicaUy considered. American Journal of Science, 

 3d ser., vol. xxvi, 1883, p. 190. 



Chambei'lin and Salisbury : The driftless area of the upper Mississippi Valley. 

 Sixth Annual Report of the U. S. Geological Survey, 1885, p. 239. 



Russell : Subaerial decay of rocks. Bulletin No. 52, U. S. Geological Survey, In 

 this bulletin will be found a full list of references up to the date of its publication in 

 1889. See also 



Shaler : The origin and natui'e of soils. Twelfth Annual Report of the U. S. Geo- 

 logical Survey, 1891, p. 219. 



Merrill : Rocks, rock-weathering, and soils, 1897, parts iii and v, and Bulletin No. 

 150, U. S. Geological Survey, 1898, p. 376. The most comprehensive and most recent 

 discussion of the various residual processes will be found in Van Hise's "Treatise on 

 metamorphism," Monograph XLVII, U. S. Geological Survey, 1904, p. 409. 



