graphs that describe the terraces of the Minnesota, and discuss the drift and its phenomena. This assignment

of these terraces to the agency of the stratified deposits of the Cretaceous does not rest on bare opinion. The Cretaceous clay was seen in outcrop near the bridge over the Vermilion near Empire City. The destructible nature of these beds causes them to be covered by loose materials, which, after the lapse of time, spread over the entire surface, and superficially appear to compose the whole substructure. The beds themselves are thus only outlined in the form of terraces.

In ascending the Minnesota valley the first point at which the Cretaceous was identified, so far as it can be without the aid of fossils, is at the Asylum farm, near St. Peter, in Nicollet county. It lies here in the water-worn openings of the Shakopee limestone. It is a white, or greenish-white deposit, holding much sand. It has a great many flinty fragments, and some siliceous limestone lumps. The latter are rounded, but the former are angular. The lumps are porous, crystalline, hard, and gray or white. No fossils can be seen, and no stratification. It seems rather to have been jammed into the openings in the Shakopee stone. Α similar exposure, presenting the same characters, occurs across the river from St. Peter, and about a mile toward Kasota, in a bluff by the side of the road.

A heavy bed of white sand, which has been described as probably belonging to the St. Peter sandstone, (p. 134), occurs on the Blue Earth and its tributaries. It is associated with an impure iron ore, and with light green shale, but its exact relation to them has not been ascertained. It may belong to the Cretaceous.

At Mankato, in Blue Earth county, a series of very interesting observations were made on the Cretaceous, throwing some light on the history of that period of submergence which brought the most of Minnesota below the ocean. Where the road to South Bend crosses the Blue Earth, on the east side of the bridge, is a cut in greenish clay, by the side of the road. This deposit of clay lies in a nook alongside the bluff of Lower Silurian, and doubtless was protected from destruction in the glacial period by that bluff. It is covered with drift, and at one place occupies a cleft in the Silurian rock, running nearly to the surface of the ground at the top of the bluff. Its position here, and as represented in the following sketch, is very deceptive. It appears very much as if in place in the Shakopee stone, but

the beds of that stone hold as low a place, horizontally, a little further to the left, as this clay. The bluff of Lower Silurian can be seen to disappear behind the clay, in some places the clay being removed so as to show the bluff at lower points than at others. There is every indication that this deposit is of small extent.

a. Shakopee Limestone.

b. Bedded, greenish clay, weathering white, but little sandy. Sandy, bedded greenish clay.

C.

d. Drift, mostly coarse fragments of Shakopee Limestone.

Passing along the right bank of the Blue Earth river from the highway to the railroad bridge, we come to a cut in the Shakopee stone. This is in the same horizon as that just described, and shows more fully the manner of superposition of the Cretaceous on the Silurian rocks. This cut is perhaps 70 feet above the river, the bank of which is composed entirely of rock, the lower portion of which is the Jordan Sandstone, and the upper, the Shakopee limestone, the latter comprising about 20 feet. In general this railroad cut shows a mixture of Cretaceous clay with the Lower Silurian, the top of the whole being thinly and irregularly covered over and chinked up with coarse drift. The Lower Silurian is more or less broken and tilted, at least the bedding seems to have been cut out into huge blocks by divisional planes, which, either by weathering or water-wearing, were widened, the blocks themselves being subsequently thrown to some extent from their horizontality, tipping in all directions. The opened cracks and seams were then filled with the Cretaceous clay, which is deposited between these loosened masses, and sometimes even to the depth of twenty feet below the general surface of the top of the rock. The clay sometimes occupies nooks and rounded angles, sometimes sheltered below heavy masses of the Silurian beds.

The clay is uniformly bedded, about horizontally, with some slope in accordance with the surface on which the sedimentation took place. But the most interesting and important feature is the condition of these old Silurian surfaces. They are rounded by the action of water, evidently waves. The cavities and porous spots are more deeply eroded, making little pits on the face of the rock; or along the lines of section of the sedimentation planes with the eroded surface, there are furrows due to the greater effect of water. The rounded surface of these huge masses of Lower Silurian is coated with a thickness of about a half inch, or an inch and a half, of iron ore, which scales off easily, and is easily broken by the hammer. While this scale of iron ore is thicker near the top and on the upper surface of the blocks, yet it runs down between the Cretaceous clay and the body of the rock, so as to prove its date older than the clay. The conclusions that must be drawn from this observation are about as follows:

1st. The Silurian rocks were long weathered, and washed clean, even waterwoan and rounded, at this place, when they went below the Cretaceous ocean. Nothing intervenes their iron-stained surfaces and the clay.

2nd. This point seems to have been on or near the ancient shore line, where the violence of the waves was great. These rounded knobs could not have received their coating of iron if constantly submerged. The iron indicates the action of atmospheric gases on iron held in solution in water, as in bog ore formation.

3rd. The Cretaceous clay here, whatever be its place, in the Cretaceous age, was deposited in a quiet ocean.

4th. This bluff, facing to the south, or south-east, like that at Mankato, indicates the approach of the Cretaceous ocean from that direction, though this may have been only a reef, or an island, just before the further submergence.

5th. There may have been, and was probably, a further deposition of clay of the Cretaceous age which was destroyed and transported by the Drift Period, at this place.

6th. The drift succeeded, and was not violent enough in its forces to disturb these tilted and waterworn masses of Lower Silurian limestone, so but that their old surfaces abut still unconformably against the Cretaceous clay.

7th. The Cretaceous sea must have advanced slowly over the Silurian rocks. These washed surfaces could not have been produced when the sea was retiring, else the Cretaceous sea would have washed out the clay. Further the clay lies directly in contact with these surfaces.

8th. The Cretaceous sea must have gone further north and east so as to deeply submerge these disturbed masses, in order to have deposited such fine sediment in their crevices.

9th. While the washed surfaces indicate a shore line, or a reef, the clay proves deep submergence.

10th. The Cretaceous sea must have retired rapidly, so as to give no opportunity to wash out the clay.

11th. These washed surfaces must have been produced as the sea was advancing.

12th. While it is certain that the Cretaceous sea advanced slowly on the land, it is not certain that it retired rapidly. The clay existing there may have been protected from the retiring shore waves by superimposed beds hundreds of feet thick-such higher beds having been subsequently removed by the drift forces; but it is likely that drift forces that were able to destroy so much Cretaceous, would also so have disturbed the Silurian masses as to leave no trace of the clay, or even to have transported away the Silurian masses themselves. Hence it is probable that there was but little Cretaceous deposited over the remaining clay, and that the sea retired rapidly.

The adjoined sketches will give a better idea of the position of the Cretaceous, with respect to the Silurian.

The Cretaceous on the Lower Silurian.

.

On Sec. 25, T. 108, R. 29, (as nearly as can be made out) is an exposure of Cretaceous Clay. The bank of the flood plain of the river is made up, almost everywhere, of sandy, more or less stratified, alluvium. But here, although having about the same hight, it is made up almost entirely of a mass of large, water-washed fragments of conglomeritic sandstone, which lie in confusion, some having fallen down into the water. They are underlain by a fine blue clay, without gravel or pebbles, belonging to the Cre taceous. Hence the sandstone or conglomerate pieces are not in situ, but pertain to the drift. Sometimes a layer of drift-pebbles and cobble-stones, about three inches thick, separates them from the clay. These large masses are 8 or 12 feet long, and 5 or 6 feet thick, and are scattered in talus over the Cretaceous clay, even into the river.

Dr. B. F. Shumard, in his report on the Minnesota valley, has mentioned an outcrop of Formation 1, capped with about 25 feet of gray, concretionary limestone, about two miles below the mouth of the Waraju, and describes it as having disseminated crystals of calcareous spar. His analysis shows it to hold 90 per cent. of carbonate of lime. A