




synchronous with the inception of the Lignite formation, it is valueless — 

_ 451. That the changed physical conditions, and thore: sae 
waters, causing this sandstone deposit, were circumstances simultan 
affecting the whole area in question, and not progressively p 
over it in any direction, appears certain. This is shown by its paleonte 
* logical similarity, and by the fact that while almost immediately suc- 
ceeded by fresh-water conditions in the east, it lies far down aan 
brackish-water and marine fossils in the west. The elevation of the 
continent following the deposition of these sandstones, taking place 
dually, and probably in a somewhat intermittent manner, and 
occasional slight relapses, brought certain areas above the surface in the | 
eastern part of the interior continental region, and also even toward the — 
west, at least as far as to the position of the present Rocky Mounts 
uplift. The sea was not perfectly excluded from the intervening shallovs 
in the east, while in the west, the waters surrounding the low-lying, — a 
forest-clad land, were essentially marine, The eastern area was, however, — 4 | 
soon converted into one characterized by shallow lakes of fresh-water, from _ a 
which the sea was permanently excluded. In and around these, the — E ae 
lignites and associated beds were formed; while westward, and “pen | 
perhaps by no distinct barrier, was a region more or less subject to 
alternations, and mingling of estuarine and marine conditions, but where | 
also the deposition of lignites was taking place i in much the same manner a 
as in the east. Everything seems to favour the belief that the deposit 
was simultaneous, as here indicated. Thus it is, that while the change — nf 
from salt to fresh-water forms, in the eastern area happens to be roughly — 














UV 
5 
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sg ier te westward as a criterion. ia ig r 
. In the eastern region, the change from Cretaceous to Tertiary — a 
life- ruin has been manifestly accelerated by the change of physical con-— 4 
ditions. In the west, a similar acceleration has also obtained, but has — 
not been so great, and while several thousand feet of strata were being a 
deposited, occasional relapses of physical conditions allowed marine | 
forms of Cretaceous aspect to re-appear. The change in type of life on on a 
which a paleontological classification is based, and which the increase an 
knowledge continually tends to show was more uniform and gradual than | 
formerly supposed, is therefore complicated over the interior area of the a ¥ 
continent by a superposed change from marine, through brackish, to 4 
fresh-water conditions ; and this did not take place simultaneously in all Ne < om 
4 bo 
‘S* (ie 
parts of the are: Be 
453. The dividing lines of geology are necessarily to a apes extent — 2 
@ 
is 5 
