68 
THE FERN FORESTS OF 
the position and outline of that particular lake 
had their immediate cause in several distinct sys- 
tems of dikes which intersect its northern shore, 
and have probably cut up the whole tract of rock 
over the space now filled by that wonderful sheet 
of fresh water in such a way as to destroy its 
continuity, to produce depressions, and gradually 
create the excavation which now forms the basin 
of the lake. How far the same causes have been 
effectual in producing the other large lakes I am 
unable to say, never having had the opportunity 
of studying their formation with the same care. 
The existence of the numerous smaller lakes 
running north and south in the State of New 
York, as the Canandaigua, Seneca, Cayuga, etc., 
is more easily accounted for. Slow and gradual 
as was the process by which all that region was 
lifted above the ocean, it was, nevertheless, ac- 
companied by powerful dislocations of the strati- 
fied deposits, as we shall see when we examine 
them with reference to the local phenomena con- 
nected with them. To these dislocations of the 
strata we owe the transverse cracks across the 
central part of New York, which needed only the 
addition of the fresh water poured into them by 
the rains to transform them into lakes. 
I shall not attempt any account of the differ- 
ences between the animals of the Devonian period 
and those of the Silurian period, because they 
