coNTmuocrs and discontinuous osars. 417 



First. A ridge formed by filling in the gaps between shorter ridges 

 ought to show the fact by its stratification. Thus far I have not observed 

 stratification of this kind. To this it may fairly be answered that the 

 number of accessible excavations in the osars is too small to be considered 

 crucial in the case. 



Second. The assumption that the glacial streams continued to flow 

 longer in the interior of the State than near the coast does not necessarily 

 imply that they were employed in osar making for a longer time. Super- 

 ficial streams could not begin to build osars till the melting reached the 

 debris in the ice. We do not know that subglacial streams of sufficient 

 size to form osars extended over all the northern osar tenitory during all 

 the- time that elapsed between the forming of the osars near the coast and 

 the final melting of the ice in the interior. This region may have been 

 in the zone of superficial streams during the earlier part of this time, until 

 the subglacial streams were extended noi'thwai-d. 



Third. When we reach northern Maine, only short ridges have thus far 

 been found. It is certain that the ice lingered longer here than it did 

 farther south, and it is at least supposable that an osar could be prolonged 

 northward as the ice receded. Instead, the appearances are as if the regions 

 of osar deposition were shifted from one place to another at the successive 

 stages of retreat — that is, not by a recession of the same osar to the extreme 

 northern part of the State, but by a transfer of osar forming to some other 

 glacial river. The hills of northern Maine would in general not be so 

 hard for osar rivers to surmount as many hills they crossed farther south. 

 But it is impossible now to determine the reasons the osars were not pro- 

 longed to the St. Lawrence-St. John watershed or beyond it, since we do 

 not yet know all the phenomena of the retreat of the ice-sheet. It is a 

 generally accepted doctrine that very deep ice invaded the Adirondacks, 

 also the Green and White mountains, from the north. This could not have 

 happened unless the valley of the St. Lawrence River were at one time 

 filled by ice as far east as the White Mountains. In a paper read before 

 the Portland Society of Natural History in 1881, I called attention to the 

 apparent diminishing of the severity of glaciation northward in Maine. 

 This was inferred from the increasing number of areas where the glaciation 

 has not obliterated the preglacial surface of weathering, also from the 

 smaller amount of attrition exhibited by the stones of the till. The latter 



MON xxxiv 27 



