DEFORMATION OF SHORE LINES. 515 



may be supposed to have reduced the ice weight and inaugurated the uplifting movement. 

 Later, when the ice front had retreated 150 to 200 miles farther, and another ice layer probably 

 1,000 feet or more in thickness had been removed, the great Algonquin uplift began. This, 

 again, continued while the ice retreated 150 miles farther, and after a second pause during the 

 time of the Nipissing Great Lakes, the uplift was resumed and has continued to or nearly to the 

 present time. The natural significance of such relations as these seems suggestive and strongly 

 favors resilience following depression by ice weight. Still, the adverse factors mentioned 

 above appear to be of equal or greater strength. 



ABSENCE OF RECENT FAULTS. 



Another thing that seems strongly favorable to the hypothesis of depression by ice weight 

 is the fact that only a few very small step faults have been found in all the region recently 

 uplifted. The. greater number of the faults so far observed run north and south in New York 

 and Vermont east of Hudson River. 1 Others have been found in Nova Scotia and elsewhere. 

 Small anticlinal folds of recent date are common in the shale around the western end of Lake 

 Ontario. Depression by ice weight would be unlikely to produce faults, because the movement 

 would take place beneath the heavy ice load, which would be in its effects equivalent to the 

 overlying load of strata that contributed to some of the strongly folded parts of the Appalachian 

 Mountains, where folding is conspicuous and faults rare. Resilience following the removal 

 of the ice would also be unlikely to produce faults, because much of it occurred beneath con- 

 siderable thickness of ice, because the upward force was widely and evenly spread over a gieat 

 area and was lacking in concentration at any particular place or along any particular line, 

 and because the forces both of depression and resilience increased with extreme slowness. 

 The phenomena of the uplifted region seem as a whole distinct and different in kind from those 

 that would be expected as normal results of the usual processes of mountain making and con- 

 tinental growth. Still, it is not certain that faulting need be strong in the initial stages of 

 folding in mountain making. 



PRE-WISCONSIN DEPRESSION AND RESILIENCE. 



The question naturally arises whether any evidence exists of depression and resilience 

 ■ in the older glacial and interglacial epochs. 



A few phenomena, such as the interglacial lake beds and the buried old land surfaces at 

 Toronto, suggest episodes of pre- Wisconsin glacial and lake history closely resembling some 

 of those associated with the Wisconsin ice sheet and the lakes of its waning phase. It seems 

 certain that if the basin of Lake Ontario in pre- Wisconsin interglacial times had been con- 

 tinuously as much depressed with reference to sea level as it was at the maximum of the 

 Wisconsin glaciation there could have been no lake beds nor any old land surfaces where 

 those at Toronto have been found, but only glacial and marine beds. 



RELATION OF ISOBASES TO THE PRE-CAMBRIAN BOUNDARY. 



The partial parallelism of the isobases to the pre-Cambrian boundary is interesting and 

 probably deeply significant, but its meaning is not yet clear. De Geer 2 found that the isobases 

 of the uplifted marine shore lines in Sweden maintain a close parallelism with the boundary of the 

 pre-Cambrian rocks, conforming even to relatively small details of that boundary. In the region 

 of the Great Lakes the isobases appear to have the same parallelism to the pre-Cambrian bound- 

 ary (line RR, fig. 14, p. 503) from the southeast end of Lake Superior to the east end of Lake 

 Ontario, runningin a nearly straight line between these points. The relations to minor details have 

 not yet been fully worked out, but so far as observed the parallelism does not appear to be so close 

 as in Sweden. Outside of the interval mentioned the isobases appear to show only a very general 



i Woodworth, J. B., Postglacial faults of eastern New York: Bull. New York State Mus. No. 107, 1907, pp. 5-28. 



2 De Geer, Gerard, Quaternary changes of level in Scandinavia: Bull. Geol. Soc. America, vol. 3, 1S92, pp. 65-66; also, On Pleistocene 

 changes of level in eastern North America: Proc. Boston Soc. Nat. Hist., vol. 25, 1S92, pp. 454-477. The important passages are quoted by Gold- 

 thwait in Bull. Geol. Soc. America, vol. 21, 1910, pp. 245-246. 



