Professor A. P. Coleman — Changes of Level. 61 



Only two explanations have been suggested, an ice dam at tlie 

 outlet of Lake Ontario, or an epeirogenic uplift of the region to the 

 north-east, like that proved by Dr. Gilbert to be now going on in 

 the Great Lakes region, followed by a sinking of the land again. 



It seems incredible that after centuries, if not thousands, of years 

 of hot summers like those of Pennsylvania the ice-sheet should still 

 be lingering at the Thousand Islands, and should even advance 

 50 miles to the south, forming a wall of ice 200 or more feet thick 

 across the powerful Interglacial St. Lawrence river. If the ice could 

 thus advance during the warm climate epoch, why did it withdraw 

 again after the climate had become cooler ? 



The improbabilities of the ice-dam theory are too great, and we 

 must turn to the other theory of an uplift to the north-east followed 

 by a depression for a satisfactory solution of the problem. Here 

 a point of interest in connection with Dr. Hoist's thesis makes its 

 appearance. If during the warm Interglacial period there was 

 a great differential uplift toward the north-east, was it not due to 

 the thawing of the Labradorean ice-sheet, supposed to have been 

 a mile or even two miles thick, and the resulting relief from 

 pressure ? 



The supposition seems probable ; and, since the water was ponded 

 back to a height more than 150 feet above the present level, may we 

 not suppose that Labrador toward the end of the time of deposit 

 stood hundreds and possibly thousands of feet higher than now ? 

 Such an elevation towards the north-east would have rendered the 

 climate of Labrador much colder ; and, perhaps aided by changes 

 in the constitution of the atmosphere or other causes, may have 

 started a fresh accumulation of ice, which as it thickened pressed 

 down the region and thus drained off the Interglacial lake and 

 allowed rivers to cut their valleys. 



Finally, the last ice-sheet, after advancing over the whole region, 

 thawed, and the slow elevation toward the north-east began again. 

 The distortion of the old Iroquois beach, as shown by the work of 

 Drs. Spencer and Gilbert, gives an idea of how much elevation has 

 been regained, and Dr. Gilbert's work in connection with lake-levels 

 proves that the process is still going on ; though the last vestige of 

 the Labradorean ice-sheet long ago disappeared. 



Apparently these changes of level are very much in arrears of the 

 changes of load, and it may take even thousands of years for the 

 earth's crust to readjust itself after a burden has been removed. 

 We must not think of the solid rock or lithosphere as a thin crust 

 resting on a molten layer, but as merging downwards into a plastic 

 layer — Dr. Murray's tectosphere — which can yield to long-applied 

 pressure and flow sluggishly to each side. After the pressure is 

 removed, it returns in an equally sluggish way. If this conception 

 of earth-movements is correct, the idea of motions due to the elasticity 

 of the crust, like a pendulum set free, as suggested by Dr. Hoist, 

 must be abandoned. The rocks forming the lithosphere are not 

 appreciably elastic, nor is the tectosphere a fluid that responds 

 quickly to changes by successive waves slowly dying out. The 



