W. J. McGee — Ovibos cavifrons from Iowa. 219 



ous element in these deltas consists of the bowlders floated 

 down in the ice floes of winter and spring, just as considerable 

 masses of gneiss are now carried into the estuarine portions of 

 the rivers by every spring freshet. Now in the Quaternary 

 delta of the Susquehanna, the bowlders are at least fifty times 

 as large as those brought down in the ice-blocks of the present ; 

 in the Patapsco and Potomac deltas, the Quaternary bowlders 

 are twenty times as large as those of to-day ; along the Rappa- 

 hannock, the ancient bowlders are ten times as large as the 

 modern ; on the James and Appomattox, the old are five times 

 larger than the new; and on the Roanoke the bowlders of the 

 delta are twice or thrice as large as those of the modern 

 alluvium. It would appear, accordingly, that the Quaternary 

 refrigeration here extended as much as, yet but little more than, 

 three or four degrees beyond the ice margin. The inference 

 is fairly harmonious, too, with the conclusions already reached 

 concerning the Quaternary climate of western United States. 

 King,* Gilbert, f and Russell:}: have indeed shown that the re- 

 frigeration of the glacial time was felt fully ten degrees beyond 

 the Cordilleran ice-margin ; but in this region the effects of 

 refrigeration were unquestionably cumulative, and the chief 

 climatal effect was increased humidity and consequent expan- 

 sion of the outletless lakes of the Great Basin, with compara- 

 tively slight increase in the local glaciers of the Sierras and on 

 the peaks of the Wasatch, and (probably) with little diminution 

 in the mean temperature. 



The recent discovery is significant, also, in that it greatly ex- 

 tends the applicability of cavifrons as a criterion for correlating 

 deposits of widely diverse genesis in widely separate localities. 

 In their classic report on the Mississippi River, Humphreys and 

 Abbot show that the so-called alluvium of the lower portion 

 of that river consists mainly of a heavy deposit of hard blue or 

 drab clay containing abundant carbonaceous matter, generally 

 covered by a slight veneer of modern alluvium, and concluded 

 that the deposit was formed u long antecedent to the present 

 epoch. "§ Hilgard designated this blue clay deposit Port Hud- 

 son,! observed that it was intercalated between the loess and the 

 Orange Sand, and inferred that it was laid down during a 

 period of submergence of the lower Mississippi region probably 

 coincident with some later stage of the glacial epoch ; and he 

 specifically supported the opinion of the engineers as to its 



*U. S. G-eol. Expl. 40th par., vol. i. Systematic Geology, 1878, 461, analytic 

 map v, 480-529. 



f 2d Ann. Rep. U. S. G-eol. Survey, 1882, 186-189. 



\ Geol. Hist, of Lake Lahontan, Monograph U. S. Geol. Survey, vol. xi, 261 ; 

 and elsewhere. 



§ Physics and Hydraulics of the Miss. River, 1876, 57-58, 91-95. 



|f This Journal, II, vol. xlviii, 1869, 332. 



