RESIDUAL TYPE 631 



somewhat as seen in volcanic^ bnt not noticeable in glacial sand (see cen- 

 ter of figure 5, plate 43). The chief characteristic by which residual 

 sand is to be distinguished from the preceding types is by the pronounced 

 weathering of all the minerals capable of such change. Many of these 

 minerals become opaque, clouded, or deepened to yellow, deep green, or 

 brown, strongly suggesting age and decay. Goodchild has pointed out 

 marked differences in the weathering of eruptive rocks and limestones, 

 depending on the amount of humidity^^ present. Under arid conditions 

 the disintegration is very largely mechanical, resulting in the splintering 

 of the cleavable minerals into fresh, angular fragments, while under 

 moist conditions there is more or less chemical decomposition, especially 

 detectable in the feldspars. In the same volume (page 444) Mackie 

 reaches the same conclusions in his paper, "Felspars in sedimentary 

 rocks as indices of climate,^^ read in 1898.^^ The resistant quartz crystal 

 fragments often are coated with iron oxide and the microscopic mount 

 gives evidence of age not seen in any other type of sand. When origi- 

 nally present in the parent rock the softer mica still remains much in evi- 

 dence, showing its ability to resist the processes of decay quite as well as 

 some of the harder minerals. Subjected to mechanical abrasion, this 

 would be the first of the common rock-forming minerals to disappear. 



When a bed of till is exposed for a long enough time to the various 

 agencies of weathering, the same ingredients take on the above character- 

 istics, and there results a subtype of sand showing a combination of 

 characters which may conveniently be referred to as residuo-glacial. 

 The pre-Kansan (N'ebraskan), Yarmouth, Sangamon, and Peorian 

 weathered zones of the various till sheets described by Leverett,^^ g^yg 

 evidence that the pebbles, and presumably the sand grains, but probably 

 to a less extent are passing into this condition. Eemoved from its asso- 

 ciations in the parent bed by slight water action^ the typical residual 

 sand is distinguished from the residuo-glacial variety by the scarcity in 

 the latter of the softer minerals and detached crystals and the relative 

 abundance of rounded granules. The typical glacial sand is to be dis- 

 tinguished from the residuo-glacial variety by the evidence in the latter 



10 Desert conditions in Britain. Transactions of tlie Edinburgh Geological Society, 

 vol. vii, 1896, p. 205. 



^ See also Merrill in Bulletin of the Geological Society of America, vol. 7, 1896, p. 

 359. 



Barrel! : Studies for students. Journal of Geology, vol. xvi, 1908, pp. 171, 176, and 

 following. 



12 Proceedings of the Iowa Academy of Sciences, vol. v, 1898, pp. 8 and 18. Journal 

 of Geology, vol. vi, 1898, pp. 171, 238, and 244. Monograph XLI, U. S. Geological Sur- 

 vey, 1902, pp. 292 and 302. American Journal of Science, vol. xxvii, 1909, p. 349. 

 See also paper by Shaler in Twelfth Annual Report of the U. S. Geological Survey, 

 1891, p. 236, 



