the Upper Cretaceous sandstones, and among those belonging to the lignitic series, this is especially noticeable. Although occurring comparatively frequently, the phenomenon cannot be regarded as a char acteristic of either of these groups. In a general way, this feature is comparable to the irregular density of the sandstones of the White River region. As soon, however, as this irregularity assumes the extreme form of concretions, we can no longer expect that great variety of fantastic figures there exhibited, because concretionary inclusions are usually shaped after the same general type.

Before closing the discussion of erosive products, I desire to point out one feature of fluviatile and pluvial erosion that is as instructive as it is beautiful, the carving of uniformly homogeneous deposits. In Colorado, ample opportunity is afforded to study this interesting phe nomenon. More, perhaps, than by any other geological group, it is exhibited by the soft shales, comparatively free from sand, of the Cretaceous formation. Frequently may be found bluffs or ridges the sides of which present a most typical miniature arrangement of hills, valleys, mountains, and cañons. What is here accomplished in a comparatively short time on so small a scale, nature's power has successfully completed in successive ages on a scale incomparably greater. Time and the never-ceasing activity of erosive influence produce results that at present fill us with astonishment and admiration. Changing from day to day, in a degree imperceptibly small to us, geological periods have been required to produce what we now see. Nothing, perhaps, expresses more aptly the lesson taught by observing the effects of erosion than the old Roman verse:

"Gutta cavat lapidem non vi,

Sed sæpe cadendo."

ART. XXXVI.-PALEONTOLOGICAL PAPERS NO. 8: REMARKS

UPON THE LARAMIE GROUP.

BY C. A. WHITE, M. D.

In other writings* I have shown that all the principal brackish-water deposits of the Western Territories are properly referable to one great group of strata which represents a period of time whose importance in the geological history of the North American continent increases with our knowledge concerning it. The members of the Laramie Group as now understood are the Judith River and Fort Union beds of the Upper Missouri River region; the Lignitic Series east of the Rocky Mountains in Colorado; the Bitter Creek Series of Southern Wyoming and adjacent parts of Northwestern Colorado, and the "Bear River Estuary Beds", together with the Evanston Coal Seriest in Bear River Valley and their equivalents in adjacent parts of Wyoming and Utah. These, at least, are the best-known members of the Laramie Group; but it has a much wider geographical extent than even the widely separated localities just referred to would indicate. Some of the known portions of this great group doubtless represent different stages of the Laramie period, but the members just designated are, as a rule, understood to represent different geographical developments of its strata with modifications of its fauna, rather than separate successive epochs of time in the geological period which is represented by the whole great group. The proof of the identity of these widely separated portions of the Laramie Group consists in the recognition of various species of fossil mollusks in all of them that are also found in some one or more of the others, thus connecting the whole by faunal continuity. Similar proof has also been obtained by Professor Cope in the discovery of certain species of vertebrate fossils in more than one of these geographical members of the Laramie Group.

The entire geographical limits of the Laramie Group are not yet fully known, but its present ascertained extent may be stated in general terms as from Southern Colorado and Utah, northward into the British Possessions; and from the meridian of the Wasatch Range, eastward, far out on to the great plains. Its extent north and south is thus known to

*See Bull. U. S. Geol. and Geog. Surv. Terr. Vol. IV, Art XXIX, and An. Rep. U. S. Geol. and Geog. Surv. Terr. for 1877.

+ Sometimes called the "Almy Mines", from the name of the small mining hamlet where the mines are located. 865

be about 1,000 miles, and east and west a maximum of not less than 500 miles. The full length of the area once occupied by the group is prob ably considerably greater than here indicated, and we may safely esti mate that it originally comprised not less than 50,000 square miles. The present range of the Rocky Mountains traverses this great area, against both flanks of which, as well as those of the Black Hills, the Laramie strata are upturned. These mountains, therefore, did not exist during the Laramie period, and the continuity of the waters of the Laramie Sea over their present site is also shown by the specific identity of aqueous molluscan fossils in its strata on both sides of those mountains.

The prevailing material of the strata, especially those of Mesozoic and Cenozoic age, in all the Western Territories, whether of marine, brackish-, or fresh-water origin, is sand; and consequently those of most of the groups have certain characteristics in common.

Not only in this general way, but in other respects also, the lithological characteristics of the Laramie Group are similar to those of the Fox Hills Group of the Cretaceous Series, upon which the former group rests, and with which, so far as is now known, it is everywhere apparently conformable; that is, it has the appearance of a widespread marine formation, consisting mainly of sandstones and sandy shales; but that it was not, like the Fox Hills Group, an open-sea deposit, is shown by its fossils. Its resemblance to the Fox Hills Group is still further increased by the presence in the latter, as well as the former, of many important beds of coal. It is true that no coal has been found in the Fox Hills Group in the Upper Missouri River region, nor in Eastern Colorado, but it is not uncommon among the strata of that group in Wyoming, Utah, and Western Colorado.

Although there is sufficient evidence that the Fox Hills Group, which immediately preceded the Laramie, was deposited in a comparatively shallow sea, the bottom of which was slowly but constantly subsiding, its waters seem to have been everywhere truly marine except in a few estuaries; and the whole area occupied by the group where it has been studied seems also to have been always and entirely submerged, except, perhaps, those surfaces upon which the coal-plants grew, and these could have been above the water-level only during the growth of that vegetation and the accumulation of its carbonized remains. The Laramie Group seems also to have been deposited in waters that were constantly shallow, and as the group has a maximum thickness of not less than 4,000 feet, the bottom must have been constantly subsiding.‡ *There must necessarily be some unconformity between these two groups in the peripheral portions of the Laramie, because, as will be shown further on, the area upon which its waters rested was cut off from the great open sea by the elevation of portions of the bottom upon which the Fox Hills deposits were made.

An interesting assemblage of fossils from a deposit of one of these estuaries has been obtained near Coalville, Utah.

Similar remarks may be made concerning all the other groups of the Western formations from the Jura Trias to the Bridger Group inclusive, as will appear further on.

In all places where the group is known, and from its base to the top, the majority of its invertebrate fossils are brackish-water forms, and yet in the same places and throughout the same vertical extent, a greater or less number of molluscan species occur that are referable to either a fresh-water or land habitat. In many instances, the fresh-water species occupy separate layers from those which contain the brackish-water forms, and alternate with them, but it is very commonly the case that both fresh- and brackish-water types are found to occupy the same layers, the con tition of the specimens of both categories being such as to forbid the supposition that either of them was drifted from elsewhere to their present places of deposit and association. For example, numerous specimens of Unio, of many species, have been found associated with equally numerous specimens of Corbula and Corbicula, a large proportion of all of which still retain both valves together in their natural position. Associated with these, and in a similarly unmutilated condition, there are other molluscan remains, the living representatives of which are respectively of fresh- and brackish-water habitat; and all of them are in such condition as to force the conclusion that they all lived together. The general prevalence of brackish-water types throughout the group, including Ostrea in abundance, Anomia quite plentiful, with occasional examples of Nuculana and Membranacea (or a closely related polyzoan), leaves no room for reasonable doubt that the prevailing condition of the Laramie Sea was saline; but the absence of true marine species proves that its waters were cut off from the open The conditions and association of species just explained show also that there must have been in certain places and at different times an alternation of greater and less saltness of its waters.

ocean.

It is well known that some species at least of certain genera of mollusks are capable of living in both brackish and fresh waters, but the evidence seems conclusive that certain forms found in the Laramie Group, the living representatives of which are respectively confined to either a fresh- or brackish-water habitat, then not only lived but thrived together in the same waters; and also that those waters were in some degree saline. This commingling of brackish- and fresh-water types is not exceptional in the Laramie Group, but quite common, yet there are layers in some places, as for example near Black Buttes, in which all, or nearly all, the Mollusca are of fresh-water type. A statement of these facts naturally suggests that this commingling of brackish- and freshwater forms took place in estuary waters, and that the strata containing them are estuary deposits. But the character and condition of the strata show that this is not the fact, or if so in any cases, they are rare and at present unknown exceptions to the rule. While there were necessarily tributary streams flowing into the Laramie Sea, and true estuaries at the mouths of at least a part of them, I do not know of a single deposit or part of one in any district or in any of the divisions of the great Laramie Group that presents the stratigraphical characteristics of an estuary deposit.

Judging from the characteristics of existing land-locked seas, it is difficult to understand clearly how fresh and brackish waters could have existed in one and the same sea in the absence of, or at a distance from, the mouths of tributary rivers; but the character of the deposits of the Laramie Sea, as well as its molluscan fauna, warrants the suggestion that many comparatively large portions of its area were, at different times and in different places, in the condition of marshes, which were only slightly raised above the general water-level, upon which fresh waters from rains accumulated, and gave congenial habitat to such members of the molluscan fauna of the period as would preferably avoid the brackish waters. This view is supported by the occasional presence of land-shells among those of branchiferous mollusks, the more common occurrence of palustral shells, the occurrence of deciduous leaves, and other fragments of vegetation, all in the same or associated strata; and also the presence of numerous beds of lignite throughout the group. It is also supported by the fact that the fossil Mollusca are found, not uniformly distributed throughout the group, either vertically or geograph ically, but to occupy small, distantly separated areas, which are not only locally restricted, but within which locally restricted areas the vertical range of the different species is limited. Admitting that such conditions prevailed, it is easy to understand how it may have happened that certain layers containing the remains of Mollusca, which could have flourished only in salt or brackish waters, as, for example, Ostrea and Anomia, are found to alternate in close succession with those containing an abundance of fresh-water species, and also with those containing a commingling of types. The conditions thus indicated would have brought the brackish- and fresh-water habitats of those Mollusca into such juxtaposition that they must have frequently encroached upon each other. This frequent encroachment, or mingling of habitats, and, no doubt, the frequent impracticability of retreat, would have had a tendency to inure at least a portion of the mollusks of each to an exist ence in the other. It is evident that many of the Laramie species were capable of such an interchange of habitat without disadvantage, and that among these were certain species of the Unionidæ, Ceriphasiidæ, and allied families.

In expressing the belief that, with the exceptions referred to, the Laramie Sea was a great body of brackish water, I have not lost sight of the fact that some living mollusks belonging to families that are regarded as of distinctively marine habitat are known to inhabit fresh waters; nor of the fact that some others which are regarded as of freshwater types are occasionally found in brackish waters. It seems impossible, however, to account for the commingling of types which we find in the Laramie strata, except by assuming that they all lived and thrived together in the same waters, as before stated.

Before leaving the discussion of the general characteristics of the Laramie Group, the existence in it of a remarkable local or regional mol