PEBBLE, GRAVEL, AND BOWLDER DEPOSITS. 853 



seems to me that this is hardly a satisfactory explanation of the very 

 abundant radiolaria which compose a considerable proportion of the 

 deposits. If Hinde's argument as to the organic origin of the cherts 

 which contain sponge spicules be correct, it probably follows that the 

 radiolarian cherts of the Franciscan series of California are also organic 

 deposits. 



Another very interesting occurrence of chert is that of the whetstones 

 of Arkansas. These are associated with clay shales" rather than lime- 

 stones. They were regarded by Griswold as fragmental, 6 and therefore 

 not belonging to the class of cherts at all, but to the novaculites. But 

 Rutley has shown that these rocks contain no evidence of clastic origin, and 

 have all the characteristics of cherts." He regards them as replacement 

 deposits of dolomite or dolomitic limestones. As this chert formation is 

 somewhat persistent, and is between shale beds which are rather impervious, 

 1 am inclined to believe that it may be largely an organic precipitate, 

 although now completely recrystallized, so as to be composed of perfectly 

 fitting granules of quartz. 



FRAGMENTAL CLASS. 

 PSEPHITE ORDER. 



The psephite order includes pebble, gravel, and bowlder deposits, 

 conglomerates, schist-conglomerates, and gneiss-psephites. (PI. VIII.) 



PEBBLE, GRAVEL, AND BOWLDER DEPOSITS. 



The psephites form below bodies of water and also on the land. 

 The chief body of water which deposits psephites is, of course, the ocean, 

 although lakes and inland seas are by no means unimportant. The 

 psephites in standing bodies of water are mainly formed along shores, 

 where the wave action is vigorous, but to some extent they occur at the 

 mouths of streams which are rapid until they reach the standing- bodies of 

 water. The psephites formed on the land are the till deposits of glaciers, 

 especially the extensive formations of continental glaciers, and the coarse 



a Griswold, L. S., Whetstones and the novaculites of Arkansas: Ann. Eept. Geol. Surv. Arkansas 

 for 1890, vol. 3, 1892, pp. 205-206. 



» Griswold, cit, pp. 168-194. 



« Rutley, Frank, On the origin of certain novaculites and quartzites: Quart. Jour. Geol. Soc. Lon- 

 don, vol. 50, 1894, pp. 377-392. 



