IV 
PREFACE. 
lead to a discussion of all the facts connected 
with the ice period, the erratic boulders, the 
drift, the formation of river systems, ancient lake 
and river terraces,- — all the phenomena, in short, 
resulting from the former presence of such im- 
mense masses of ice and their subsequent disap- 
pearance. These questions have been chiefly 
studied on the European continent, where the 
broken character of the country, intersected in 
every direction by mountain chains, presents nu- 
merous centres of dispersion for glaciers. Owing 
to the extensive land surfaces on this continent, 
the same set of facts presents quite a different 
aspect here and in the Old World ; and I hope 
that the facilities I have had for tracing the gla- 
cial phenomena in America may enable me to 
throw some new light on this subject. 
L. AGASSIZ. 
Cambridge, November 29th, 1865. 
