310 • GLACIAL GEAVELS OF MAINE. 



the bubble, could be pushed forward, to be discharged into the first crevasse 

 or cavity formed in lee of an obstruction. By some such process the basal 

 waters are able to maintain a precarious and much-interrupted passage 

 beneath the ice. 



At North Dixmont and elsewhere osars that are somewhat transverse 

 to the glaciation are stratified monoclinally, the dip being toward the lee 

 side, as if the advance of the ice continually closed up the stoss side of the 

 enlarging channel and left a con-esponding opening on the lee side. 



GENESIS AND MAINTENANCE OF SUBGLACIAL AND ENGLACIAL CHANNELS. 



Of this intricate subject our definite knowledge is phenomenal and 

 general rather than causal and detailed. Rivers are known to flow within 

 or beneath the ice. The surface waters plunge down crevasses and disap- 

 pear. These facts are well known. But as to the parts of the work 

 wrought respectively by the ice and the water, these and many similar 

 questions can be argued, but not determined by direct observation. 



No other means than crevasses for the passage of superficial waters 

 beneath a sheet of ice covering all the land has been discovered. If inter- 

 stitial water reaches the ground through granular snow and consolidating 

 ice, or if surface pools melt their way to the bottom, these processes would 

 hardly merit naming as exceptions to the foregoing rule, since they could 

 supply so small an amount of water. We have, then, to consider the ice- 

 sheet as one of the rocks which surface waters penetrate, as they do other 

 rocks, along a system of joints and crevices of wonderful complexity till 

 they reach the earth or the bottom of the crevices. Thus in the first 

 instance the ice itself provides the means for the descent of the waters. It 

 is at the escape of the waters horizontally that difficulties begin. Gen- 

 erally the streams are longitudinal, while the greater part of the crevasses 

 are transverse. The transverse crevasses break up the glacier into parallel 

 blocks or prismoidal slices, each of which, judging from surface appear- 

 ances, is capable of acting as a dam to. hold back the waters above it. 

 Where the ice is broken longitudinally or, as not infrequently happens, 

 alike transversely, longitudinally, and obliquely, the waters find no diffi- 

 culty in making their way by zigzags through the labyrinth of crevasses. 

 But it is known that surface streams often sink into the ice far above the 

 greatljr shattered portions of the ice-sheet. There must be subglacial or 



