were undoubtedly scooped or worn out of the granite rocks by glacial action. They afford a splendid example of what Prof. Ramsay, the eminent geologist of England, calls " Rock Basins," the origin of which he has so graphically explained in his volume on the "Physical Geography and Geology of Great Britain." In the Upper Arkansas valley there seems to have existed in glacial times, one immense glacier, rising to the height of 1000 to 1500 feet on the mountain sides, and filling up the entire valley, with tongues or branches extending up the numerous side cañons. A description of this remarkable district may be condensed from the Report of the U. S. Geol. and Geog. Survey of the Territories for 1873 and 1874. The Arkansas valley, from its head in Tennessee pass to the point where the river cuts through the Front or Colorado range and opens out into the plains, has been worn out of the granite mass to a great extent. The origin of this valley is mostly due to erosion. From the crest of the Park range, on the east side of the Arkansas river, to that of the Wasatch on the west, the average distance in a straight line must be at least ten or fifteen miles, and the average elevation above the water level of the river 1500 feet. It is probable that this great space was, at no very ancient period, filled with one vast glacier, which doubtless performed the greater part of the grinding up of the rocks and the wearing out of the valley. The glacier-worn sides of the gorges, point strongly to that conclusion.

But in this brief article we must confine ourselves mostly to the limited district, the valley of Lake Fork, in which the Twin lakes are located, the subject of the illustration. The valley of Lake creek is filled with the morainal deposits for which both sides of the Wasatch range of mountains are so remarkable. It would seem that the great glacial force moved here in a direction a little south of east, inasmuch as the mass of the detrital matter is heaped up on the south side. The two lakes are about three hundred and fifty yards apart, with a small stream flowing from the upper into the lower, about twenty feet in width. The interval is made up of worn detrital matter, but over it and around both lakes, are mounds or oblong ridges of drift; and scattered over the surface, are masses of granite, coarse in texture, with crystals of feldspar, one and two inches in diameter, aggregated together. The rock has the appearance of a feldspathic breccia. The lower lake is about two and a-half miles in length

and one and a-half miles in width, the upper lake is one mile in length and a-half mile in width. As we have stated before, the greatest depth was found to be seventy to seventy-six feet. The Lake creek rises about twelve or fifteen miles away, at the crest of the Wasatch range, and flows through a deep gorge or cañon, with signs of glacial erosion its entire length, and as it issues from the mountains into the main valley, has become a considerable stream. These lakes are really expansions or basins in this stream and a part of it. That these lakes have been slowly diminishing in area, we know by the land bordering on both of them. Above the Upper Twin lake, there is a half mile in width of boggy meadow, which at no distant period must have been covered by the lake. At the head of the valley, or where the gorge begins, there is a sort of natural bridge, where the stream has worn a narrow channel through the rocks. At the summit the gorge is about eight feet wide, and in it a huge boulder has lodged. The stream rushes down its steep, narrow, winding channel with great force. On the north side there is a huge boulder just ready to topple off into the channel, which is fifty feet in diameter. On the sides of the channel are several most remarkable rounded cavities worn in, like pot holes, six to ten feet in diameter. One of these occurs twenty feet above the water level of the creek at the present time. The worn rocks, or roches moutonnes, are most admirably shown everywhere, and portions crop out in the bottom of the valley to indicate the force as well as the extent of the erosion. It is quite possible that if all the debris could be stripped off the gorge and valley, the grooved or scratched surfaces would be apparent. One immense mountain mass on the north side seems to have resisted the eroding forces, so that from base to summit, a heighth of one thousand feet, it is smooth, like enamel. The great glacier which must have filled up the channel, has probably been obstructed, in its slow downward movement, by this projecting point of the mountain. The great branch glaciers of Lake creek must have been at least 1500 feet thick. The valley or gorge is of nearly uniform width, about one-fourth of a mile, and the glacier must have ploughed its way along, removing a great thickness of the gneissic rocks on either side and on the bottom, rounded remnants of which can be seen cropping everywhere from the detritus. About six miles above Twin lakes, in a straight line, Lake creek forks, one branch extending up toward the north-west, and the other south-west.