1917] 



Moody: Breccias of the Mariposa Formation 



403 



usually exhibits sharply individualized beds of coarse and fine material 

 alternating in a variable sequence. 



The importance of glaciers as agents of transportation and deposi- 

 tion both in early as well as late geologic time is now generally 

 accepted. Shaler and Davis. 20 as early as 1881, recognized, beside 

 Quarternary glaciation, evidence of glacial action in the Superga 

 Miocene of Italy, in the Eocene of Switzerland, in the Chalk of 

 England, in the Jurassic of Scotland, in the Triassic of eastern 

 United States, and in the Permian, Subcarboniferous (Mississippian) , 

 Devonian, upper Silurian and Cambrian in various areas throughout 

 the earth. Their views have been somewhat modified by later work, 

 but deposits referable to glacial agencies have since been found of 

 even greater age than they then surmised. No general exception then 

 can be taken to the possibility of glaciation in the Mariposa. 



The Permo-Carboniferous Talchir group of India described by 

 "White, 21 Oldham and others is a typical glacial deposit. It consists 

 of 500-800 feet of clays, fine silts, boulders, sandy shales, conglomer- 

 ates, and soft sandstones. The included fragments are everywhere 

 subangular, and are mostly derived from distant localities. Large 

 boulders often lie embedded in silt too fine to admit any explanation 

 except that of deposition from floating ice. Smoothed and striated 

 stones are frequent in the unsorted masses. 



Boulder beds very similar to those of India occur in New South 

 Wales, Australia, and in South Africa. The Dwyka Conglomerate 

 in the latter area is now well known in the literature of glaciology. 

 A good description of this formation is given by Mellor. 22 The rock 

 is usually bluish or green and consists of fragments of a number of 

 mineral species. Within the mud matrix are included a vast number 

 of boulders and pebbles of a variety of rocks, scattered irregularly and 

 indiscriminately throughout its mass. Many of the boulders are 

 rounded ; many others are flattened and striated in two or more direc- 

 tions. Lenticular patches of shales and mudstones, apparently de- 

 posited in pockets in the conglomerate, consist of the finest glacial 

 mud. Professor Davis 23 remarks that since no marine deposits are 

 associated with the formation, which extends over thousands of miles, 

 he considers that the Dwyka ice was a continental sheet analogous to 

 the Pleistocene glaciers of the northern hemisphere. Beautifully 



20 Shaler and Davis, W. M., Glaciers, p. 102, 1881. 



21 White, C. D., American Geologist, vol. 3, p. 299, 1889. 



22 Mellor, E. T., Quar. Jour. Geol. Soc, vol. 61, pp. 683-86, 1905. 

 2-'i Davis, W. M., Bull. Geol. Soc. Amer., vol. 17, p. 413. 



