often called "barns". Such appellations as "natural bridge", "gate", "gateway", "tunnel", and others, designate each a definite class of forms. They are too generally understood to require any discussion as to ety. mology and comparative applicability. Occurrences of this character have always been invested by man with more or less mythical interest, and many a legend is told which stands in the most intimate connection therewith.

ISOLATED FORMS.

As "isolated forms" we may regard such that vary in their distribution and mode of occurrence from the classes above enumerated. Taking for instance the trachytic conglomerates: we may regard the occurrence of grotesque forms within that group as a characteristic thereof. It is an essential feature, and one that may readily be employed in determining this recognition. In contradistinction thereto, isolated products of erosion are such that occur only sporadically in geognostic groups or formations where they would not be expected. Some peculiar circumstances may combine to produce such results, and in vain may similar forms be looked for at other localities within the same horizons. From the nature of the case, it is evident that the isolated monumental products will occur comparatively rarely, and that they will show a great diversity of composition and shape. Only a few such instances will be mentioned from Colorado as comprising the most. prominent representatives of this class. Extensive erosion within certain regions will necessarily result in the formation of objects that would appropriately be placed under this head, but for our purposes it will be entirely sufficient to refer to but a few of them.

LIZARD'S HEAD.

The Mount Wilson group is located a short distance west of San Miguel Lake, in west longitude 107° 59′ and north latitude 37° 50′ Descending by the Bear Creek trail from the divide between Rio Ani. mas and San Miguel drainage, we see before us a steep, downward slope which abruptly terminates in the valley of the last-named river. Two thousand feet below us lies the placid sheet of water which receives its name from the river. Looking beyond it toward the northwest, we see the mountain mass of the Wilson group rising high up in bold relief. An elevation of 14,280 feet is reached by the main peak, the summit being nearly a mile higher than the lake. Prominent in the mountain group we notice a "needle" standing near its eastern edge. From a distance it appears insignificant, but we can easily determine that it must be of large proportions in order to be seen at all. As we approach closer, we find that a comparatively regular pedestal has been formed, supporting an enormous monolith. Steep slopes lead up to its base, broken often by vertical walls. From this base rises a gigantic rock

*Compare Annual Report for 1874, fig. 2, page 207.

column, 290 feet in height, while its diameter amounts to from C0 to 80 feet. Its isolated position permits it to be seen for a long distance, and its elevation-13,160 feet above sea-level at the summit of shaft—renders it an excellent landmark for all the lower country adjoining.

Both the rocks composing the Wilson group and those which the monolith-Lizard's Head-exhibits, are of volcanic origin. In former geological periods enormous masses of sedimentary and volcanic material have been eroded and transported from that region. It seems possible that a former connection existed between the ridge now sup porting Lizard's Head and the main volcanic group farther east. No surface connection exists at present, however. All that remains in the immediate vicinity of them is the huge monolith. During the period of the great erosion, valleys were cut into the rocks and ridges were gradually carved away so as to become narrower and shorter. Probably the disturbances produced by eruptions of volcanic material, and, more particularly, the phenomena accompanying them, rendered the rocks of that region less capable of resisting such powerful agents of demolition as were then employed. It may be observed that the trachytes composing Lizard's Head show a certain development of columnar structure. This structure is almost invariably accompanied by basal fracture-planes. By this means, erosion will be enabled to attack such portions more successfully. A process of undermining will result in the falling of overhanging portions. Owing to the columnar arrangement of the integral parts composing a hill or bluff, the faces produced by such falling will be quite or nearly vertical. In this manner, fluviatile erosion can produce, from such material, a type of form which is represented by Lizard's Head. Had the erosion continued on at the same level, the entire mass must have succumbed. Increasing width and depth of the excavated valleys, however, caused the waters to sink. Thereby the same species of erosion was produced along the sides of that portion which now forms the "pedestal", but the column remained intact. This appears to be the only way of accounting for the existence of Lizard's Head. It is not a dike or intruded volcanic product, subsequent to the main eruptions, but a portion of the regular flows, large masses of which are still preserved not far distant.

Similar in shape are the forms resulting from a partial breaking-down of mural products of erosion. Their arrangement, however, and the character of the rocks composing them, will admit of their ready identi fication.

Another important group of isolated forms of erosion comprises such that are produced by local inclusions of essentially foreign material. Concretions may be contained quite frequently in shales and sandstones. Those to which we have special reference here are harder, resisting erosion and disintegration more effectually than the rocks containing them. Forms similar to those of the monuments may be produced by a gradual wearing-away of the portions adjacent to concretions. Among

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 phenomenon. 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 Liguitic 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 Utab. 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 verte. brate 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 every where 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 for mations from the Jura Trias to the Bridger Group inclusive, as will appear further on.