338 



THE EEGOLITH 



bility. The volcanic dusts are as a rule siliceous, more nearly 

 allied to the acid potash rocks than to the basalts. 



The analyses given below show the chemical nature of (I) a 

 jSne, white, almost flour-like pumice dust from Harlan County, 

 Nebraska, and (II) of dune sands from the Pamlico Peninsula, 

 North Carolina. This last is described^ as a tolerably fine, 

 nearly white sand consisting of smooth, well-rounded grains, 

 mainly quartz, but containing also occasional shell fragments 

 and black granules of iron ore. 



Chemical Analyses of Volcanic Dust and Dune Sand 



OONSTITtJENTS 



Silica (SiOa) . . • 

 Alumina (AlgOs) *| 

 Iroa oxide (Fe^Os) j 

 liime (CaO) . . . 

 Magnesia (MgO) . 

 Potash (K2O) . . 

 Soda (Na20) . . . 

 Sulphuric acid (SO2) 

 Ignition. . , . . 



92.12% 

 5.29 



1.13 

 0.03 

 0.64 

 0.35 

 0.33 

 0.60 



(4) Glacial Deposits. — Under this name are included those 

 drift deposits which are the product mainly of glacial action, 

 though their immediate deposition may have been brought about 

 in part through the instrumentality of water. The strictly 

 aqueo-glaeial materials have been noted under the head of 

 alluvial deposits. 



Allusion has been already made to the manner in which gla- 

 ciers erode and transport. During a comparatively recent 

 period in geologic history, there appears to have come over a 

 portion of North America a gradual lowering of the normal 

 temperature or increase in the annual precipitation, or perhaps 

 both, until the condition of affairs existing in northern Green- 

 land prevailed as far south as the 39th parallel of north lati- 

 tude. Now whether the ice sheet extended at any one time 

 over the area outlined below or whether there were periods 

 of advancement and retreat; whether the glaciation was pro- 



^ Geology of North Carolina, Vol. I, 1875, pp. 182-183. 



