159 
surface as the Boston Basin presents, and it became essentially 
stationary, or was subject to slight local movements only ; and 
presenting still a fairly continuous sheet of solid iee, with 
surface streams and crevasses, the conditions were especially 
favorable for the formation of normal glacial potholes by 
stationary moulins. Seventh, the contours and distribution of 
the modified drift in this region indicate that the ice-sheet 
became, later, very ragged, being frayed out at the margin 
and divided into numerous detached masses, so that important 
surface streams and moulins ceased to exist. In conclusion, 
attention may be called once more to the fact that the glacial 
potholes prove by their very existence that there has been no 
appreciable glaciation of the rocks in which they occur since 
they were formed. In other words, they are a record of a 
time when the ice-sheet still covered the land, but had become 
nearly or quite stationary. 
It may be remarked in this connection, also, that while the 
till in this region is found to lie almost universally upon strongly 
glaciated rock surfaces, the deposits of modified drift repose in 
part upon glaciated, but very largely upon unglaciated and 
distinctly water-worn surfaces. It is a common circumstance 
also to find the protruding and stoss portions of ledges from 
which the sand and gravel have been removed more or less 
distinetly glaciated, while all the depressed and lee surfaces are 
beautifully sculptured by water, showing many incipient pot- 
holes connected with winding channels and separated by 
smoothly hummocky and undulating surfaces. The glacial 
striae often end abruptly at the margins of these depressed areas, 
showing very distinctly the co-operation or simultaneous action 
of water and ice at a time when the ice had but little erosive 
power, and proving very clearly, also, that the water was 
subglacial. 
MODIFIED DRIFT AND TERRACES. 
It is generally conceded that the recession and final dis- 
appearance of the ice-sheet was attended by, and probably 
