594 PRINCIPLES OF STRATIGRAPHY 



"What would be the opinion of a geologist, viewing such an enor- 

 mous collection of bones, of all kinds of animals and of all ages, 

 thus embedded in one thick earthy mass? Would he not attribute 

 it to a flood having swept over the surface of the land, rather than 

 to the common order of things?" (Darwin-17, Chapter vii.) 

 These droughts are of frequent occurrence and seem to show an 

 approximate periodicity of about fifteen years. 



Distances to Which Material May Be Carried by Rivers. Wade 

 (55), in his study of the distribution of the gravels derived from 

 the hills of igneous rock facing the western border of the Red 

 Sea, comes to some interesting conclusions regarding the distance 

 to which pebbles may be transported by subaerial agencies. On the 

 eastern side of these ranges the fragments have traveled only a 

 short distance, lying still in the plains at the foot of the hills. On 

 the western side of the Red Sea ranges the gravels appear to have 

 been carried along the main wadis and old lines of drainage often to 

 great distances. "The pebbles have traveled in more or less westerly 

 directions down the wadis, into the important north-and-south wadi 

 which continues the line of the Nile north from Quena. They are 

 abundant, for some distance north and south of Quena. Thence 

 they have been carried northward down the Nile Valley. . . 

 The most interesting occurrences of the rocks, which the Survey 

 geologists say 'could only come from the Red Sea' are at Heluan, 

 a, few miles south of Cairo . . . and in the Delta itself, where 

 they were found in the Royal Society's boring at Zagazig. Thus 

 these rocks have been river-borne for at least 400 miles." (Wade- 

 ^^•.244.) The age of these gravels is Pleistocenic. 



Extensive deposits of apparently river-borne pebbles are found 

 in many older formations. In the Trias of England they have been 

 found 300 miles from their source. The Pottsville conglomerate 

 series of eastern North America extends northwestward for a dis- 

 tance of 400 miles or more, the pebbles of the series throughout be- 

 ing derived from the Appalachians on the southeast. 



Purity and Rounding of Fluviatile Deposits. On the whole, the 

 material of a subaerial fan is very heterogeneous, but by prolonged 

 reworking and the sorting action of running water a considerable 

 assortment into kinds as well as sizes of grain may be effected. 

 In dry climates the disintegration of granitic rocks is not accom- 

 panied- by extensive decomposition. Fresh feldspar crystals will 

 remain, and these may be rounded by wind attrition. (Wade-55.) 

 In moister climates, on the other hand, feldspar will decay, forming 

 kaolin or laterite, and by deflation or by flotation these products of 

 finer grain will be swept away. 



