gO THE JOURNAL OF GEOLOGY. 
it has moved are so many and so distinctive, and stand in such 
nicely adjusted relations to one another, that it hardly seems 
credible that any second agency or combination of agencies 
could produce results so similar as to be mistaken for them, if 
all the foregoing characteristics and relations are clearly devel- 
oped and open to observation. It is not to be understood that 
all the marks of glacier deposits, or all the phenomena accom- 
panying them, are always present at every point, or within any 
circumscribed area where glacier ice has been. The study of 
considerable areas of glacier deposits may sometimes be neces- 
sary for the recognition of all the features and relations referred 
to. Some, or even many of them may altogether fail of develop- 
ment in a given locality. Those which are developed may be 
but feeble. In a limited area of drift where relations are not dis- 
cernible, or where many of the various characteristics referred to 
above are but poorly developed, or where they are not open to 
observation, glacier deposits might be confused with those pro- 
duced by certain other agencies, especially if the region in ques- 
tion be one where glacier ice is not known to have been. If a 
glacier deposit be very ancient, it may have lost, by decay, most 
of its diagnostic characteristics; or, since its origin, it may have 
suffered alteration, with the effacement of its distinctively glacial 
marks, at the hands of some geological agent other than ice. It 
is not to be understood, therefore, that glacier deposits can always 
and everywhere be recognized at sight, especially in a region 
where glaciers do not exist, and where they are not known to 
have existed. But it is confidently believed that any series of 
typically developed glacial deposits which have suffered so little 
change that they still preserve the characteristics which the ice 
impressed upon them, cannot fail to give evidence of their 
origin, if their characteristics and relations are open to observa- 
tion. 
If glacier ice alone be responsible for the drift, it is necessary 
to suppose that the whole of the drift-covered area was over- 
spread by an ice-sheet. Alpine glaciers are so small and so 
connected with mountains, that studying them alone, it seems 
