BASAL FURROWS AS STREAM TUNNELS. 309 



this happened only while the latest scratches were being made. An earlier 

 series of scratches went up and over and down the slope of the rock with- 

 out distinguishable break of continuity. These scratches do not date from 

 the time when ice was deepest, but are themselves deflected from the direc- 

 tion of general glaciation, jet at the time they were made the ice could 

 flow down into depressions without leaving caves beneath it. The scratches 

 on the tops of the highest hills date from the time the ice was deepest, and 

 scratches parallel to this direction are remarkable for the depth of the 

 depressions they go down into and the abruptness of the slopes they are 

 able to follow. A fair inference is that the furrows or hollows left beneath 

 the ice while passing over uneven ground, bowlders, and other obstacles 

 are a feature of thin glaciers. Manj^ observers have seen such furrows in 

 the lower surface of the ice where it flowed over bowlders, but their obser- 

 vations were necessarily made in the crevassed portions of glaciers near 

 the extremities. Such furrows must fill up by inward flow of the ice, 

 and the rate would depend on pressure, etc. 



The hypothesis that basal furrows and lee cavities have helped to form 

 subglacial stream tunnels has some quasi support from certain field phe- 

 nomena. Thus in the coast region the g-ravels are often found on the tops 

 of low hills, but in sxich places it is probable that crevasses would be formed, 

 and these might aid in the formation of tunnels far more than the basal 

 cavities. None of the hillside eskers have been seen to originate from 

 bowlders or sharp peaks of rock, or to have such in their courses. The 

 bosses of rock that are sometimes fovnid in the course of an osar river ai"e 

 so low and broad that only very short cavities would form in their lee. 

 And since such cavities were largest near the extremity of the ice, where 

 crevasses were most numerous and sufficed to carry off" the waters, we must 

 infer that basal furrows and caves were of little use in establishing stream 

 tunnels. 



Another conceivable sort of basal cavity attracts attention as a possi- 

 bility. Under unbroken ice the Avater of basal melting would be pressed 

 sidewise from where there is greater pressure to where there is less pressure, 

 and collect beneath the ice. Since water is practically incompressible, such 

 a water-filled cavity can not collapse in one part without a corresponding 

 expansion in another. It would in some respects be the analogue of the 

 air bubble in water, though not owing its shape to surface tension, and, like 



