

‘CRETACEOUS AND TERTIARY —CONDITIONS OF DEPOSIT. 163 
the succeeding periods of depression, could supply a great quantity of 
ready formed detritus. 
384. The earlier beds included in the Lignite Tertiary series, while 
showing a continued progression of the physical changes operating 
during the later Cretaceous periods, evidence the elevation of much 
dry land; and though these deposits are supposed to have passed entirely 
across the present Rocky Mountain region in some places, * still, about 
_ this time a pretty continuous barrier of land must have been formed 
there. The northern current still passed along a narrow trough on the 
eastern margin of this iand, and piled up local and irregular sandbanks, 
while numerous molluscs of shallow and brackish-water types, assisted in 
the formation of the deposits. At the same time, or very shortly after- 
ward, further to the east, much finer sediments were being laid down 
with considerable regularity; and only in the very lowest layers of these 
do we find salt or brackish-water species of shells. The source of the 
vast quantity of fine argilo-arenaceous material of the Lignite Tertiary,— 
much of which must have been depositsd in land-locked basins,—is a ques- 
tion of very great difficulty, unless, indeed, we may suppose that some 
parts of the Cretaceous series were already undergoing denudation by 
rain and rivers. If the Rocky Mountain region became an actual land 
barrier about the beginning of the Tertiary, the later Cretaceous deposits 
of the west coast, should present a more coarsely detrital aspect than those 
of the western part of the interior of the continent, while those of the 
earlier Tertiary, should be relatively finer in thejfirst locality. For, dur- 
ing the former period, the shallow banks and bars would supply material 
to the south-western drift, while during the latter, they would prevent the 
access of other sediment than that derived from their own waste. 
385. It is unnecessary here to recapitulate, in giving an account of 
the lithological character of the beds of the Lignite Tertiary, which are 
fully described in connection with the various sections on preceding pages. 
In some parts of the formation, however, the lignite beds themselves 
constitute a not unimportant part of the thickness of the strata. In one 
of the Souris River sections, in a total thickness exposed of 57 feet, the 
lignite amounts to 17 feet, and is disposed in seven distinct beds, separ- 
ated by clays and sands. The lignites themselves comparatively seldom 
show intercalations of shale or detrital matter, and their ash is found, as 
a rule, to consist of very fine whitish, to pale yellowish or reddish, earthy 

*Dr. Hayden. Geological Report Yellowstone and Missouri Expedition, p. 5, U. S. Geol. Surv, 
Territ., 1867-69, p. 70, &c. | I p. o eol. Surv, 

