376 GLACIAL GEAVELS OF MAINE. 



3. The deltas of different streams are sometimes confluent. This lact 

 still more enlarges the continuous areas of supposed floating ice. 



4. Unless the till were, in New England, confined to the very bottom 

 of the ice, the till contained in the ice above the limit of supposed melting 

 would greatlj^ increase the specific gravity of the floating ice. It is not 

 proved that till-containing ice could be sustained by flotation in so shallow 

 water. Out in the Gulf of Maine at a great depth the buoyancy of the 

 purer upper ice would enable a thick sheet of till-bearing ice to float. But 

 the largest of the glacial miarine deltas are situated near the contour of 230 

 feet, where the water would be less than 100 feet deen. Only a thin sheet 

 of pure ice could float under these circumstances. 



5. In the case of the Silsby Plains in Aui'ora, mentioned above, we 

 have proof of tidal action, and most of the deltas spread outward so rapidly 

 as to indicate the cooperation of the tides in the work of strewing the 

 sediment of the glacial rivers up and down their valleys and along the coast 

 and out to sea. Tidal currents in the open sea would do this work. It is 

 uncertain what would be the action of the tides on the sediments beneath 

 floating ice, since the free space beneath the ice would change with the 

 state of the tide and the thickness of ice. 



6. The rise and fall of the tides would cause a strain on the central 

 parts of the supposed undermelted ice such that from time to time small 

 bergs would break off and float away to sea. Thus, even if the bottom 

 ice were melted over so large areas the upper ice would soon disappear and 

 the supposed cave would become a part of the open sea. 



The difficulties of the hypothesis of large areas of undermelted ice 

 are so great that the hypothesis that the marine deltas were laid down over 

 the bottom of the open sea I consider by far the best interpretation, though 

 the margins of the ice channels would be undermelted, just as they were 

 on the land. 



SYSTEMS OF DISCONTHSTUOUS OSAES. 



In this class a number of short ridges, often plain-like, have a linear 

 arrangement and other relations such that they are regarded as having been 

 deposited by the same glacial river. These systems have nearly the same 

 general directions as the continuous osars, and their topographical relations 

 are substantially the same. The osars as they approach the coast become 

 discontinuous, like the systems now to be described. In the case of the 



