G. F. Wright — Continuity of the Glacial Period. 181 



In Professor Salisbury's last Report upon the surface geology of 

 New Jersey, be arrives at nearly the same conclusion with Pro- 

 fessor Lewis, namely, that there was during the earlier glaciation 

 a subsidence of the eastern coast in that region to the extent of 

 at least 130 feet. In an examination which I made last sum- 

 mer, in company with Mr. Ernest Yolk, of the gravel terraces 

 west of Trenton, ]ST. J., and extending from Fallsington to 

 Yardleyville, Pa., the facts elicited seem to show that the de- 

 position was continuous and comparatively rapid from the 

 gravels at a level of about 1T0 feet near Yardleyville down to 

 the valley at Fallsington, where the level of the terrace is but 

 slightly above that of Trenton. The gravel rests upon the 

 gentle slope in a nearly continuous blanket, and not in the 

 irregular manner which must have been the case if it had been 

 working down from a higher elevation over a slope that was 

 gradually formed by the lowering of a broad valley through 

 undercutting of the solid rock. 



The evidence brought to light during the past year thus 

 seems in the clearest manner to remove many of the arguments 

 which have been adduced to prove an extremely long inter- 

 glacial episode and the consequent discontinuity of the epochs. 

 Several specially important papers, however, have been pre- 

 sented during the year aiming to show that the lapse of time 

 between the maximum of glaciation and the formation of Pro- 

 fessor Chamberlin's " terminal moraine of the Second Glacial 

 epoch " is much longer than the period which extends from 

 the formation of the second moraine to the present. Mr. 

 Leverett (the value of whose extensive observations is becom- 

 ing more and more apparent), in a paper before the Geological 

 Society of America in August last, adduced much cogent evi- 

 dence to show that the present valley of Rock River in Illinois 

 is different from the preglacial valley, and that where it now 

 runs through the area of the older drift the erosion is so great 

 that it calls for seven or eight times as long a period as that 

 demanded by the postglacial erosion of the Niagara River or 

 by the postglacial work of the Mississippi below the Falls of 

 St. Anthony. Mr. Oscar H. Hershey,* also, adduces similar 

 evidence from the area of the older drift occupied by several 

 small tributaries of Rock River in Stevenson County, Illinois. 

 The minimum age of those portions of the gorges of these 

 streams which have been formed since the occupation of the 

 region by ice, is estimated by Mr. Hershey to be 50,000 years. 

 Mr. Leverett also adduces the great amount of erosion in the 

 till which covers Sangamon County and the region farther 

 south in Illinois, as compared with that north and east, to 

 prove that the older drift is several times older than the newer. 



* Am. Geol., vol. xii, p. 314 seq. • 



