TESTS OP SUBGLACIAL OE SUPEEFICIAL DEPOSITION. 427 



or rapids. Another way of accounting for them would be by the action of 

 ice dams such as would naturally form when the spring- floods began to 

 break up the ice and snow that had gathered in the open channel during 

 the preceding winter. As the waters poured over the dam, the unusual 

 velocity would erode sediments that had previously been deposited in the 

 chamiel, and they would be piled up a short distance below. On this 

 theory there ought to be a gap in the ridge just north of the cone of gravel. 

 Such gaps are found north of the "Pinnacle" at Pittsfield, also north of 

 several similar enlargements of the Exeter Mills-Hermon osar. I have no 

 sections showing the nature of the stratification at these places. If the 

 stratification of the cones is quaquaversal, it will favor other theories rather 

 than the ice-gorge theory. 



On the whole, we must conclude that the pinnacles do not afford a 

 satisfactory test as to whether the osars were deposited in subglacial or 

 superglacial channels. 



BEGAD AND MASSIVE ENLARGEMENTS. 



Such are the so-called "mountains" of Greenbush. On the one 

 theory subglacial streams poured into a gradually enlarging lake. On the 

 other a very broad and deep enlargement was gradually made in the super- 

 ficial channel. It is only the case of the pinnacle on a large scale. But in 

 this as in many other cases the rival theories may have to compromise. 

 A surface stream may have poured into a pool, like many of the streams of 

 the Greenland ice, and have escaped as a subglacial stream. 



I can discover here no satisfactory test for the two theories. 



RETICULATED RIDGES. 



Reference is nere made to the plexus of ridges into which an osar often 

 expands. 



Superficial channels can become filled and new ones formed, as every 

 river delta pi'oves, and as we see exemplified on every hillside during the 

 melting of the snow and ice in spring. A subglacial channel can also 

 become clogged by sediment, and it is easy to conceive circumstances such 

 that a new channel could be more readily formed than the old one could 

 be enlarged. The conditions under which the reticulated ridges were 

 formed will be more fully discussed hereafter. For the present I only 



