164 IOWA ACADEMY OF SCIENCE 



really represent great continental delta fans, but also to out- 

 line the probable climatic conditions which existed at the time 

 they were formed by comparing them with similar recent and 

 near recent deposits of known origin. 



Studies such as those made by Sherzer 4 , who found upon 

 examining recent sand grains formed by various agencies that 

 each type possessed characteristics to a certain degree of its 

 own, also promise to be of great value in deciphering the his- 

 tory of the ancient sediments. For instance there are strong 

 reasons for suspecting that certain sandstone formations made 

 up of sand grains possessing all the characteristics of recent 

 wind blown sand are of eolion origin, or at least consist of 

 eolion sands reworked by the sea as it transgressed upon the land. 



Similarly Walther and Huntington and others by their de- 

 scriptions of the characteristics of modern desert deposits have 

 contributed valuable data which already have been applied in 

 interpreting the history of the sediments of the past. Thus, 

 wind carved pebbles similar in every way to those described 

 by Walther and others from the Libyan desert have been 

 found, according to Grabau 5 "in the pre-Cambric Torridon 

 sandstone of Scotland, the basal Cambric sands of Sweden, the 

 Rothliegende of Germany, the Buntersandstein of Thuringia 

 and elsewhere" thereby suggesting strongly the existence of 

 desert conditions at the time these beds were formed. In like 

 manner a type of cross -bedding shown by "Walther to be char- 

 acteristic of the modern sand dunes of the deserts of Egypt, and 

 observed by Huntington in Persia, Transcaspia and Chinese 

 Turkestan has been observed by Huntington in certain Mesozoic 

 sandstones of Utah and by Grabau and Sherzer in the Sylvania 

 sandstone of Silurian age, of Michigan 7 . 



But in spite of the great advancement of physical strati- 

 graphy within recent years resulting from the field study of 

 sediments, we may expect even greater advances in the future, 

 especially as the result of more detailed examination of the sedi- 

 ments with the aid of the microscope. Here lies a great field 

 almost untouched, although its possibilities have been shown 

 by the studies of Sorby, Cayeux, Mackie, G. S. Rogers, Gold- 



*Bull. Geol. Soc. America, Vol. 21, 1910, pp. C25-GC2. 



Principles of Stratigraphy, p. 54. 



"Bull. Geol. Soc. America, Vol. 18, 1907, p. 351. 



7 Micb. Geol. and Biol. Survey, Pub. 2, Geol. Series 1, 1910, p. CI ff. 



