212 SURFACE GEOLOGY. 
arly favorable to the formation of great glacial masses of ice, 
i.e. a broad evaporating surface of warm water swept by 
westerly winds that carried all suspended moisture immedi- 
ately on to a mountain belt, which served as a sufficient con- 
denser. | 
This, at least, may be positively asserted in regard to the 
agency of ice in the excavation of the lake basins, that their 
bottoms and sides wherever exposed to observation, if com- 
posed of resistant materials, bear indisputable evidence of 
ice action, proving that these basins were filled by moving 
glaciers in the last ice period if never before, and that part, 
at least, of the erosion by which they were formed is due to 
these glaciers. 
No other agent than glacial ice, as it seems to me is capa- 
ble of excavating broad, deep, boat-shaped basins, like those 
which hold our lakes. 
If the elevation of temperature and retreat northward of 
the glaciers of the lake basins were not uniform and contin- 
uous, but alternated with periods of repose, we should find 
these periods marked by excavated basins, each of which 
would serve to measure the reach of the glacier at the time 
of its formation, the lowest basin being the oldest, the others 
formed in succession afterwards. Such a cause would be 
sufficient to account for any local expansions of the troughs 
of the old ice rivers. 
Where glaciers flow down from highlands on to a plain or 
into the sea, the excavating action of the ice mass must ter- 
minate somewhat abruptly in the formation of a basin-like 
cavity, beyond which would be a rim of rock, with whatever 
of débris the glacier has brought down to form a terminal 
moraine. 
= When glaciers reach the sea, the great weight of the ice 
mass must plough up the sea bottom out to the point where 
the greater gravity of water lifts the ice from its bed, and 
bears it away as an iceberg. 
If it is true, as the facts I have cited indicate, that our 
