AQUEOUS TYPES 633 



specific gravity, while if these properties vary considerably there would 

 occur a corresponding variation in the relative sizes of the granules de- 

 posited in the same bed. The experiments of Sorby, reported in the 

 above cited paper, indicated that a current of about 6 inches per second 

 is sufficient to slowly drift along granules of common sand having a 

 diameter of about a hundredth of an inch. No rippling of the sand was 

 produced until the velocity of the current was somewhat greater (page 

 180). The "angle of rest/^ in water, of sand averaging about .05 inch 

 (ranging from .03 to .07 inch), he found to be about 41 degrees when 

 simply coming to rest, but about 49 degrees before giving way after hav- 

 ing been at rest. For smaller sized granules the angles were less, being 

 34 and 36 degrees for grains averaging .01 inch and 30 and 33 degrees 

 for sand averaging about .003 inch in diameter (page 174). The author 

 describes (pages 186-188) what he terms "drift-bedding" as resulting 

 when sand drifted by a current reaches relatively deeper water, by which 

 the velocity is checked and the sand deposited at the angle of rest in a 

 bed, the thickness of which corresponds to the increased depth. When 

 deposited by currents of water the angle of rest is considerably reduced. 

 As a result of the steep angles of rest, sand deposited under rapidly shift- 

 ing currents may show very marked cross-bedding. In the case of all 

 classes of aqueous deposits there is always the possibility of the introduc- 

 tion of marine or fresh water forms of animal or plant remains. No 

 especial significance, however, can be attached to their absence, since a 

 bed of rapidly accumulating or shifting sand is unfavorable for life and 

 the permeability of a sand deposit favors the decay and removal of 

 organic forms. In the case of granules practically insoluble in ordinary 

 water, the shaping of the same is due mainly to their mutual abrasion 

 while in motion. Based on a study of the modern and ancient sands of 

 Great Britain, Mackie in 1897 developed a formula expressing the capac- 

 ity of sand grains to become rounded in water, the factors involved being 

 the size, specific gravity, hardness, and distance traveled.^^ It was found 

 that the larger grains showed the most evidence of rounding; that where 

 the grains were of about equal size the heavier were the more rounded; 

 that where size and specific gravity were about the same the softer showed 

 the more rounding. In those cases in which the source of the sand could 

 be determined it was shown that those sands which had traveled the 

 farthest were the most rounded. Assuming the size, shape, and distance 



1^ Mackie : On the laws that govern the rounding of particles of sand. Transactions 

 of the Edinburgh Geological Society, vol. vii, 1897, p. 300. See also paper by Good- 

 child (loc. cit.), 1896, p. 208. Also On the constitution and history of grits and sand- 

 stones, by Phillips. Quarterly Journal of the Geological Society of London, vol. xxxvii, 

 1881, p. 21. 



