92 
THE FERN FORESTS OF 
America, as well as a large part of its Western 
territory, were still to be added. But although 
its central portion held its ground and was never 
submerged again, yet the continent was slowdy 
subsiding during the middle geological periods, 
so that, instead of enlarging gradually by the in- 
crease of deposits, its limits remained much the 
same. 
This accounts for the very scanty traces to be 
found in America of the secondary deposits ; for 
the Permian, Triassic, and Jurassic beds, instead 
of being raised to form successive shores, along 
which their deposits could be accumulated in reg- 
ular sequence, as had been the case with the 
Azoic, Silurian, and Devonian deposits in the 
northern part of the United States, were con- 
stantly sinking, so that the Triassic settled above 
the Permian, tho Jurassic abovo the Triassic, and 
so on, each set of strata thus covering over and 
concealing the preceding one. Though we find 
tho stratified rocks of these periods cropping out 
here and there, where some violent disturbance 
or tho abrading action of water has torn asunder 
or worn away the overlying strata, yet we never 
find them consecutively over any oxtensive re- 
gion ; and it is not till the Cretaceous and earlier 
Tertiary periods that wo find again a regular suc- 
cession of deposits around the shores of the con- 
tinent, marking its present outlines. It is, then, 
