50 GEOLOGICAL SURVEY OF THE TERRITORIES. 
plains. These limits indicate for the ancient Laramie sea a length of 
about one thousand miles north and south, and a maximum width of not 
less than five hundred miles. Its real dimensions were no doubt greater 
than those here indicated, especially its length; and we may safely as- 
sume that this great brackish-water sea had an area of not less than 
fifty thousand square miles. The present range of the Rocky Mount- 
ains, which has been entirely raised as a mountain range since the close 
of the Laramie period, traverses almost the entire length of this great 
area, and far the greater part of the other extensive and numerous dis- 
placements which the strata of the different geological ages have suffered 
within that great area have also taken place since all the Laramie strata 
were deposited, although some of those changes thus especially referred to 
began before the close of the Laramie period. Itis for these reasons that 
the great region within which the strata of the Laramie Group are found, 
however much they may be now displaced, is assumed to have been 
entirely covered by the Laramie sea during the whole of that period, 
except such portions as rose above the surface by stratigraphical dis- 
placements before its close, as before intimated. * 
The invertebrate fauna of the Laramie Group consists almost wholly 
of brackish-water, fresh-water, and land mollusea. Species belonging 
to all three of these categories are often found commingled in the same 
strata, but it is also often the case that certain strata, sometimes only 
thin layers, which contain the fresh-water and land mollusks alternate 
with those which contain the brackish-water species. With the excep- 
tion of one species of Axinca, one of Nuculana, and one or two of Odonto- 
basis, no species usually regarded as of marine types have been found 
in any of the strata of the Laramie Group;t and even these are doubtless 
such as became inured to a brackish-water habitat as the waters were 
progressively freshened, just as certain marine types of fishes and other 
aqueous animals have become inured to brackish and even fresh waters 
both in past geological epochs and at the present time. Such brackish- 
water types, however, as Ostrea, Anomia, Corbula, Corbicula, and Neritina 
characterize the Laramie strata throughout their whole geographical 
extent, and from top to bottom of the group. Indeed this brackish- 
water fauna is the great distinguishing feature of the Laramie Group, 
so far as its remains of invertebrate life are concerned, and all the genera 
just named, except the last (the representatives of which are compara- 
tively rare), are abundantly represented from the base to the top, or 
near the top, of the group. 
All the species of fresh-water and land mollusea which prevailed during 
the Laramie period seem to have ceased with the disappearance of their 
contemporary brackish-water forms, although they were succeeded by 
other fresh-water and land species; so that we find it convenient and 
proper to fix the latest limit of the Laramie Group where the brackish- 
water forms disappear, especially so because no brackish-water forms are 
known in all that great western region in strata later than those of the 
Laramie Group. It is, however, quite proable that some species of the 
fresh-water and land mollusca of the Laramie period continued to live 
after its waters were fully freshened and all brackish-water forms had 
disappeared, but no instances of a commingling of those Laramie fresh 
* The frequent alternation of strata in the Laramie Group which contain only fresh- 
water molluscan forms with those which contain brackish-water forms, and the proba- 
ble conditions which produced that result are discussed in the two publications cited 
on the preceding page. 
t The reference of Znoceramus and other marine forms to strata of this group (= Point 
_ of Rocks Group of Powell), in chapter iii, Powell’s Report on the Geology of the Uinta 
Mountains, was the result of error in stratigraphical determinations. 
