IMPRESSIONS OF THE PAST 41 
tween the deposits being so short that the 
tracks of the big Dinosaurs extend through 
several sheets of stone ; while again there was a 
period of drouth when the shore became so dry 
and firm as to retain but a single shallow im- 
pression. 
Something of the wealth of animal life that 
roamed about this estuary may be gathered 
from the number of different footprints re- 
corded on the sands, and these are so many and 
so varied that Professor Hitchcock in two ex- 
tensive reports enumerated over 150 species, 
representing various groups of animals. One 
little point must, however, be borne in mind, 
that mere size is no sure indication of differ- 
ences in dealing with reptiles, for these long- 
lived creatures grow almost continuously 
throughout life, so that one animal even may 
have left his footprints over and over in as- 
sorted sizes from one end of the valley to the 
other. 
_ The slab shown in Fig. 7 is a remarkably 
fine example of these Connecticut River foot- 
prints; it shows in relief forty-eight tracks of 
the animal called Brontozoum sillimanium and 
