Wie H. M. BANNISTER 
considering only the peripheral portion of the ice-sheet and not 
its greatest development. The length of the interglacial epoch 
must in any case enter as a very important element in our esti- 
mates of the total time required for the deposition of the driit, 
and the limited data from exposures are at best more suggestive 
than definite in the information they convey as to this point. 
With these divergent views as to geologic time it would not 
appear as if the glacier theory afforded a very satisfactory basis 
for reasoning upon this particular phase of the subject. There 
certainly seem to be decided difficulties in utilizing the drift 
phenomena for the measurement of geologic time, each appar- 
ently possible solution of the difficulty seeming to present still 
more impossible problems. Some of the estimates made appear 
to be compromises, therefore, or alternatives, accepted only as 
better than something else. Thus Prestwich and Wright find it 
easier to limit the duration of the glacial period than to admit 
the possibility of man having existed 80,000 years on the earth, 
or that the fauna or flora of today could possibly be the same as 
that of 240,000 years ago. The elements of individual prepos- 
session and mental idiosyncrasy enter largely into the considera- 
tion of scientific questions, and all the more into such as this 
where the chances of legitimate difference of opinion are 
so ample. 
It is the object of this paper to call attention to the method 
of calculating geologic time by the transportation of erratics, a 
method that has up to the present time hardly received the 
attention, it deserves im the literature of they subjects a liteissat 
first sight a little remarkable that this should be the case. That 
some at least of those who have alluded to it appear to have dis- 
credited its value is also remarkable, as any method that adds 
any degree of certainty to our estimates ought to be regarded 
as a boon to science. While, as I shall attempt to demonstrate, 
it has its value even with the rather indefinite notions we have 
hitherto had as to the flow of continental glaciers, recent 
researches by Chamberlin and others on the Greenland ice-sheet 
have added very much to its importance and applicability. We 
