THE GEOLOGICAI MIDDLE AGE. 
175 
still very incomplete, I assert that just where the 
direct sequence of geological deposits is needed 
for this evidonce, we have it. The Jurassic beds, 
without a single modern scaly Fish, are in imme- 
diate contact with the Cretaceous beds, in which 
the Fishes of that kind are proportionately almost 
as numerous as they are now ; and between these 
two sets of deposits there is not a trace of any 
transition or intermediate form to unite the rep- 
tilian Fishes of the Jurassic with the common 
Fishes of the Cretaceous times. Again, the Cre- 
taceous beds in which the crowded banks of Ru- 
distes, so singular and unique in form, first make 
their appearance,, follow immediately upon those 
in which all the Bivalves are of an entirely differ- 
ent character. In short, the deposits of this year 
along any sea-coast or at the mouth of any of our 
rivers do not follow more directly upon those of 
last year than do these successive sets of beds of 
past ages follow upon each other. In making 
these statements, I do not forget the immense 
length of the geological periods ; on the contrary, 
I fully accede to it, and believe that it is more 
likely to have been underrated than overstated. 
But let it be increased a thousand-fold, the fact 
remains, that these new types occur commonly at 
the dividing line where one period joins the next, 
just on the margin of both. 
For years I have collected daily among some 
