epoch been occupied by a much larger river, which had formed the outlet of a great lake, embracing the lake Winnipeg and receiving the drainage of its basin, and extending as far south as the present Lac Travers at the source of the Minnesota River. This ancient river, if its volume were as great in proportion to area as that of the Mississippi above the falls of Saint Anthony, probably equaled Niagara in volume, and would have been sufficient to prevent the formation of such excessive accumulation of débris in its course, such as the Mississippi is now making below the junction of the Minnesota River, although it is probable that the material brought into it from the near proximity of the falls of Saint Anthony, would have had the usual effect of somewhat increasing the slope and shoalness just below its junction, and decreas ing the slope and increasing the average depth just above. Accepting, then, the conclusion which I have elsewhere made reasonable, that the drainage of the Winnipeg basin was formerly along the Minnesota and Mississippi below the junction, then when the flow of water from that great northern basin ceased there would no longer be the volume of water necessary to remove the deposits brought by the affluents into a channel of too great capacity for the requirements of the new conditions. In building the railroad bridge across the mouth of the Minnesota, Mr. Shepard, the engineer, made borings to ascertain the character of the foundations. A rod was forced down about 60 feet, at which distance a stratum was reached so yielding that the rod's weight would be barely supported. He did not endeavor to probe further, for just before reaching this soft layer he passed through a harder one of sufficient thickness and resistance to support his structure. This bridge is a work of minor character. This shows that the deposit in the old valley exceeds 60 feet at this point. The Minnesota being a muddy stream, its fine silt has much filled the lake which the Mississippi débris at its mouth caused. The débris and material brought over and from the Falls of Saint Anthony has been carried downward by the water, gradually accumu lating and filling the valley as it advanced. One should here stop and consider the manner in which river deposits are made. The finest clay is so easily mingled with the water that the slightest disturbance in the fluid as it moves even with the gentle currents, is sufficient to keep it from settling down until the still water is reached. The amount of this on the Upper Mississippi is comparatively very small. Such materials as can be moved only by the swiftest currents are rolled over along the bottom, gradually diminishing in size by friction, and furnishing smaller particles susceptible of being thus moved by feebler currents. The resulting material is pebbles, gravel, and sand, the fine material, such as clay and vegetable matter, being all washed out. When deposits of gravel or sand are made of materials moved along the bottom, it takes place as soon as the current slackens, as it must do on reaching a place having larger sectional area. The deposit is sudden, and the material is all taken up in diminishing the section, until the velocity of transportation is restored; then deposition continues immediately in front of the last deposit. Such deposition, therefore, does not extend laterally from the course of the current into any contiguous dead water, as depositions from water holding a clayey or vegetable matter would. Thus in the valley of the Mississippi, the lakes alongside the river's course are deeper than the river, which has continued to raise its bed by deposits of sand after the lakes were cut off from its current. From the Falls of Saint Anthony down to the Saint Croix River the Mississippi Valley receives no considerable tributary. The Saint Croix comes from a region of trap rock now furnishing little or no large and heavy sedimentary matter. The result has been that the Mississippi's deposit of sand and gravel has been thrown across its mouth, holding back its water and forming the Saint Croix Lake. At low water, while the depth of the Mississippi at the juncture is 23 feet, the depth in Lake Saint Croix is 25 feet. It is not probable that 25 feet depth represents the amount of filling of the ancient valley at this point, because the lake itself must have been somewhat shoaled with fine deposits of clay and vegetable matter. The next considerable tributary to the Mississippi Valley is the Chippewa. This, entering at right angles with a steep river slope and a probable high-water volume of at least 40,000 cubic feet per second, comes from a region inexhaustibly supplied with siliceous sand and gravel containing a considerable of the heavy magnetic sand, whose oxidation often cements the other sand deposits.* It brings quantities of these materials, which spread out below, give a very steep slope to the Mississippi River and very bad shoals for navigation. Lying just above this deposit is Lake Pepin, which it completely accounts for. The reason this lake has not been filled up by the Mississippi above is that the supply of sand from the Chippewa is so great as to raise the level more rapidly than the filling above can keep pace with. The Chippewa from the left bank pushes its sand-bar out, so as to confine the outlet to the lake to the opposite shore. There is an observaable relation between the condition of the lake and the deposits of the Chippewa. The deepening of the waters by the deposit of Chippewa sands is felt at low water sometimes as far up as the mouth of the Saint Croix, when floods in the Chippewa make these deposits large, and on the other hand, in times of droughts the waters of the lake cut the outlet deeper, and lower its level, so that the shoal water is moved down the river two or three miles below the Saint Croix. If we follow the Mississippi down we find similar conditions produced by the Wisconsin River as by the Chippewa; that is, a great increase of the slope and shoaling of the river below the junction, with gentler slopes, deep water, and lake-like aspect above. There would probably have been a large lake here if the discharge of Black River, just above, had not silted it up. Another instance is afforded by the damming-back effect of the Missis sippi deposit at the mouth of the Illinois River, making it at low water almost like a lake up to La Salle. Lake Pepin must therefore be regarded as due to the deposit by the Chippewa of heavy coarse sediment into the valley of an ancient and larger river. This view may be strengthened further by the following considerations: It lies immediately in the course of the main valley above an important tributary. In this respect it agrees with Lac-quiparle, on the Minnesota, just above the Lac qui-parle River; with another lake on the same valley just above the Yellow Earth River; with Big Stone Lake in the same valley, just above Whetstone River; with Lake Traverse, which is formed by deposits from a stream at each end, and this empties sometimes in both directions. It agrees in this relation * The analysis of the soil on this part of the Chippewa River (the Yellow Banks) gives 93 per cent. of insoluble matter, which is chiefly white sand, with only 2 per cent. of organic matter, less than 4 per cent. of soluble saline matter, consisting chiefly of oxide of iron and alumina with only a trace of calcareous earth.-(Owen's Report, p. 56.) with the lakes on the Qui-Appellee, which all lie just above a consider ble tributary, and with like lakes on the Upper Fox of Lake Winnebago. This constant relation seems unmistakably one of cause and effect. Valley now filling up.-From what has been stated above, it is clear hat the river valley in the part we have considered is not now being deepened by erosions, but, on the contrary, is filling up, and it appears to be doing so all along its lower course except at the rapids. Recent drainage of Lake Winnipeg southward.-As I have stated in previous reports, I regard the ancient river draining the Winnipeg southward by the Minnesota and Mississippi Valley as existing subsequent to the glacial deposits. This is based upon the fact that the river's course is cut through those deposits, as shown by the banks in many places from Lake Traverse, in Minnesota, to Warsaw, in Illinois, and that the ancient bed of Lake Winnipeg is free from glacial deposits, and exhibits only the silt-deposit since made by the ancient lake itself. Valley formed since the glaciers began to retire.-It also seems most probable that the ancient valley itself, as a whole, was formed in the region of glacial deposits, partly during the period this great field of ice was receding, and partly since it left the ancient Mississippi basin, for the following reasons: When this ice-period was on the increase, its southern margin must have been gradually advancing in this region, crushing down and planing off the ridges and filling the ravines and water-courses with the débris not only of the neighboring rocks, but with the great mass of bard rocks and other material brought from regions far to the north. There seems a probability that much of the present Upper Mississippi basin had previously been for long ages exposed only to erosions of streams and of the atmosphere, so that it was probably much cut up and fissured, as we see in regions farther west, where no glacial action has occurred. It must have been an easy matter then for the glacier to have thoroughly filled up all the valleys and ravines, leaving the surface every where of the well-known rounded hill and basin forms of the drift regions. Wherever the glacial scratchings are preserved, their uniform directions indicate a massive movement to the southwest quite independent of all influence of underly ing inequalities. The water which flowed from them would seek the the first lowest line and excavate its course without regard to the nature of the older stratified rocks buried beneath the glacial deposits, and such seems to have been the case, for the valley takes a great variety of courses, running about northward at Saint Paul, due west at Rock Island, and its directions fill every azimuth in different parts from northeast around by south to west. To the old stratified rocks its course seems to have no relation, now cutting across an anticlinal, then following the strike in one direction and again in the opposite one. How the valley was formed.-At Saint Paul, on the Mississippi, and in the Minnesota above, are the banks of an ancient water course when at such higher level than now that the river-bed was the magnesian lime. stone rock, the same as that of the Mississippi, just above the Falls of Saint Anthony. The existing channel of the ancient valley has probably been formed by a cataract in the great river, similar to that at Saint Anthony. This river is sustained by the high islands of rock in the valley of the Minnesota, being remains of strata once continuous across it. These high islands also exist below in the Mississippi, such as Baru Bluff, at the head of Lake Pepin, and the Trempeleau hills. Some of these detached bluffs may have been formed by bends approaching each cther by erosions gradually forming a neck and cutting it off. One such, nearly completed, is seen in the Dalles of the Chippewa River. The |