WAREEN UPHAM, ESQ., ON CAUSES OF THE ICE AGE. 221 



lati'on of the Pleistocene ice- sheets was preceded and caused 

 bv great epeirogenic elevation deserve careful attention. 

 The first consists in an approximate identity of level Avith 

 that of to-day having been held by many drift-bearing areas 

 at a time very shortly preceding their glaciation. This is 

 clearly known to have been true of Great Britain and of 

 New England. Near Boston, Mass., for example, my obser- 

 vations of fragments of marine shells in the till of drumlins 

 in or adjoining the liarbour prove for that tract a preglacijd 

 height closely the same as now at so late a time that the 

 molluscan famia, of which we have a considerable representa- 

 tion, comprised only species now living. In respect to tin's 

 objection, it must be acknowledged that the preglacial high 

 elevation which I think these areas experienced was geo- 

 logically very short. With the steep gradients of the 

 Hudson, of the streams which formed the now submerged 

 channels on the Californian coast, and of the Congo, these 

 rivers, if allowed a long time frtr erosion, must have formed 

 even longer and broader valleys than the still very impres- 

 sive troughs which are now found on these submarine 

 continental slopes. But the duration of the epeirogenic 

 uplift of these areas on the border of the glacirition for the 

 Hudson, beyond it for the Californian rivers, and near the 

 equator in western Africa, can scarcely be compared in its 

 brevity with the prolonged high altitude held during late 

 Tertiary and early Quaternary time by the Scandinavian 

 peninsula and by all the northern coasts of North America 

 from ]\Iaine and Pug<^t Sound to the great Arctic Archipelago 

 and Greenland. The abundant long and branching fjords 

 of these northern regions, and tlif' wide and deep chainiels 

 dividing the many large and small islands north of this 

 continent attest a very long time of preglacial high elevation 

 there. At the time of cnlinination of the long continued 

 i\nd slowly increasing uplifts at the north, they seem to have 

 extended dining a short epoch far to the south, coincident 

 with the formation of ice-sheets in high latitudes But 

 when these lands became depressed and the ice burden of 

 the glaciated countries was removed, they in some instances, 

 as in Great Britain and New England, returned very nearly 

 to their original levels, beautifully illustrating the natural 

 condition of equilibrium of the earth's crust, which Button 

 has named isostasi/, that when not subjected to special and 

 exceptional stresses it acts as if floating on a heavier plastic 

 and mobile interior. 



