STUDIES FOR STUDENTS 169 



shaped retains its form indefinitely. Thus the St. Peter sandstone, 

 whose grains were originally ground under desert winds, was 

 spread by the Ordovician sea, and again supplied the material 

 for the Sylvania sandstone of Michigan. Either of these sand- 

 stones could furnish desert sand to breccias of several different 

 types. 



Bajada breccia is of local derivation; far- traveled stones from 

 distant sources are not expected, yet the extent and complexity of 

 the feeding-mountains may give the breccia great lithological 

 heterogeneity. Ancient breccias are not necessarily associated 

 with the buried mountains or uplands which supplied their waste. 

 Such elevations may have been destroyed in the building of the 

 bajada or by later denudation. 



The Permian breccias of the Midlands, England, are now com- 

 monly regarded as of bajada origin. 1 They occur in rudely strati- 

 fied, wedge-shaped masses, some more than 200 feet thick at one 

 end and thinning out within four to eight miles. Beds of breccia 

 are inters tra tiffed with current-laid sandstone and with marl, into 

 which they graduate both vertically and horizontally. The frag- 

 ments embrace a large variety of rocks and are now considered of 

 local derivation. They are angular and subangular, more or less 

 water- worn, but are never well rounded. Half a foot is a common 

 measure of their size where they are largest. The matrix is cal- 

 careous or sandy. 



Certain coarse breccias, intercalated with thin sandstone layers, 

 in the Esmeralda formation of Nevada have been classified as 

 detrital-slope breccias by Turner. 2 Examples well known to 

 American students are the bajada breccias of the Newark forma- 

 tion. 



Glacial breccia. — Subaerial glacier deposits are certainly to be 

 classified as breccia, but their characteristics are so well known and 

 so easily recognized, as a rule, that no description is considered 

 needful. 



1 R. D. Oldham, Quar. Jour. Geol. Soc. London, L (1894), 463-70; W. W. King, 

 ibid., LV (1899), 97-128; T. G. Bonney, ibid., LVIII (1902), 185-203. 



2 H. W. Turner, "Geology of Silver Peak Quadrangle," Bull. Geol. Soc. Am., 

 XX, 245. 



