148 The American Geologist. September, iS93 
glacier can be supplied with material for erosive work. It 
may carry or drag along the loose material which it finds in 
it- path : or it may rend rocks asunder wherever a place of 
entry is found: or it may obtain material from the rock itself 
by scouring it with the cutting tools already supplied. When 
the two first sources fail the third must fail also, for the ice 
of itself can have no appreciable effect upon solid rock. 
The supply of loose materials ceases when the zone of dis- 
integration is passed, and the second source must fail soon 
after this. The erosive action of ice is not to roughen the 
surface over which it moves, but to round, smooth and polish 
it. and consequently to lessen tin- possibility of obtaining a 
supply of cutting tools. As the period of ice occupancy of a 
land continues, the power of erosion must diminish and finally 
amount to almost nothing. The ice will then slide over 
rounded surfaces practically without any destructive effect 
and the streams will then issue from its front with a very 
slight load of sediment. 
These statements should, perhaps, be qualified somewhat, 
since unconsolidated strata will continue to furnish detritus 
as long as they are exposed to the scouring action of ice: but 
even this supply will be limited, and, given time, its effects 
will disappear. For New England, where the facts were ob- 
served upon which these deductions are based, this disturbing 
factor is of very little importance and may. therefore, be 
neglected. 
Using these facts as a basis, we have a means of determin- 
ing the topographic age of a glaciated country by a study of 
its topographic form and glacial deposit. A young glaciated 
region, one subjected to glacial erosion for a brief period, 
should be littered with glacial drift, probably composed to a 
greater or less degree, of the products of disintegration. In 
a later stage, that of maturity, these deposits would be in 
greater measure, perhaps entirely, composed of fresher rock 
fragments distributed in greatest abundance near the pe- 
riphery of the ice-sheet. During old age the country would be 
nearly free from deposits, and the topography would consist 
of a series of rounded, polished hills of glacial erosion. The 
first stage would be brief, the second much longer, and the 
passage to extreme old age one of very slow development. 
