vature;" in short, "that currents of electricity alone could produce the sudden vacuum by removing the air above." His book is valuable for the many facts and indefatigable labor in its pages, but utterly fails to establish the supposition that magnetism and electricity account for all weather phenomena. The very arguments adduced, every fact brought forward to overthrow the theories of Espy, Redfield, and others, goes to establish them. As for instance, he denies that the wind blows from all points towards the center of a storm; but admits that it flows in from either side in a curved path. Very well. Now suppose a storm stands still and the wind blows towards the center all round. This is what he expects to see made out. But give that storm a rapid onward motion and things will be slightly disarranged. Add to this the air rushing down in the middle and up the sides, and a great complexity of motions will take place. Almost every objection urged against the theory, even the curved path of the inflowing lateral air currents are capable of a mathematical demonstration by the laws of motion and in accordance with the theories of Espy and Redfield. The former of these supposes the gyratory motion of the tornado; the latter the aspiratory, or that the air currents flow in and up without any whirl. We think the true theory is a combination of both these, as advanced in a former part of this paper. In other words, the air flows in from all sides and begins to flow upwards in straight lines, but being subject to the law of motion, which makes fluids running through a funnel assume a rotary motion, it is soon thrown into a gyratory motion, which increases as it gathers fresh momentum.

Electricity is not the cause, but the fearful accompaniment of a storm; and in the tornado its fury is greatly increased by the friction of the whirl and the rapid condensation of the vapor above.

GEOLOGY OF A SECTION OF THE ROCK

RIVER VALLEY,

FROM OREGON, IN OGLE COUNTY, TO STERLING, IN WHITESIDE COUNTY.

Read before the Illinois Natural History Society, June 27th, 1860.

By OLIVER EVERETT, M. D., of Dixon, Illinois.

My object, in this paper, is to give some of the results of observations made by me upon the geology of the Rock River Valley, in Lee county, and a part of Ogle and Whiteside counties, or from about Oregon, in Ogle county, to Sterling, in Whiteside county. The surface in this part of the country is much more rolling, or undulating, than in most parts of the State. This is particularly the case in the upper portion of the section alluded to, in Ogle county and a part of Lee county, where it is frequently cut up into deep ravines, on the sides of which the underlying rocks are often exposed to view; and the banks of Rock River and its tributaries frequently present bold, perpendicular bluffs of rock, from fifty to two hundred feet high, thus giving a tolerably good opportunity for geological investigations. These features are most prominent in the region of one member of the geological series of which I shall hereafter speak, viz: the Upper or St. Peter's Sandstone. In another section, where the Trenton Limerock underlies the drift, there are frequently found deep pits in the ground. These pits are generally more or less circular, and are from one to two or three rods in diameter, at the surface of the ground, and run to a point below. They are from ten to twenty and sometimes thirty feet deep, and have, evidently, been produced by the earth, in these places, falling into and being carried away by subterranean streams of water in the loose rock below.

Below Dixon, although the surface is considerably undulating, it is not so abruptly broken by deep ravines, and the prairies generally slope gradually to the banks of the river, seldom exposing the rocks at all. Below Dixon there is very little woodland along the banks of the river, while above, between Dixon and Oregon, a considerable portion of the country along the river is covered with

timber. The timber is not generally of very heavy growth, although, in some places, on the bottom lands, it is quite large. It consists of the various species of oak and hickory common to the State, the black and white walnut, the sugar and silver-leaved maple, box elder, (Negundo accrifolium,) sycamore, the red and white elm, hackberry, ash, linden, cottonwood, etc. The red cedar, the white pine, the ground hemlock, (Taxus Americana,) the black and the paper or canoe birch, (Betula lenta and Betula papyracea,) are found on the extreme verge of the rocks overhanging the river and creeks, beyond the reach of the prairie fires. All these last mentioned species, except the red cedar, are found, as far as I have observed, only upon the bluffs formed by the St. Peter's Sandstone. We should naturally expect to find on a soil produced from the disintegration of this sandstone, some plants which are not common to the rich alluvial and clayey soils of a large portion of the State. Accordingly I have found several species not included in Dr. Lapham's Catalogue, and some of them not in the additional lists subsequently made by Drs. Brendell and Bebb, and which I presume are not often found in other parts of the State. Among which I might name two species of vaccinium, the Aretostaphylos uvauisi, Lupenu perrennis, Campanula rotundifolia, Talinum teretifolium, Lobelia kalmii, Cerastium oblongifolium, Linaria canadensis, Fragaria vesca, and the Viola lanceolata, which grows on the borders of ponds, or in wet places in this sandy soil.

The drift formation, through this section, is probably not so thick nor so uniform in depth as in most parts of the State There are many things in relation to it which have peculiar interest, but my object in this paper is to speak of the rock beneath it.

There is, in this section of about thirty miles of the Rock River Valley, a pretty good opportunity to study several important members of the lower Silurian system, and some of the lowest strata of the upper Silurian series.

Commencing at Oregon, with the St. Peter's Sandstone, and ascending the geological scale, as we go down the river, we find the Buff Limestone, (of Owen,) the Trenton Limestone, the Galena Limestone, and the shales, etc., representing the Hudson River group of the lower Silurian system, and the Niagara Limestone of the upper Silurian series.

ST. PETER'S SANDSTONE.

The lowest rock which we find in the section under consideration is the Upper or St. Peter's Sandstone. It is the prevailing rock along the river, from a mile above Oregon to about three miles below Grand De Tour, a distance of thirteen or fourteen miles. On the north-west side of the river, I think that in no place does this rock appear on the surface more than two or three miles from the river. On the south-west side it extends several miles back from the river. I should think that the thickness of this rock could not be less than two hundred feet, and probably more. The country

It

where this rock prevails is characterized by great unevenness. is frequently cut up into deep and sharp ravines, and, in many places, there are bold, precipitous bluffs, from one to two hundred feet high. I have not often found these bluffs capped with the Trenton Limestone, as spoken of by Prof. Hall as being the case in Iowa. In many places this sandstone is interspersed with numerous horizontal bands or layers of iron, or sandstone so impregnated and cemented with the oxide of iron, as to be very firm and resisting. These layers are from less than half an inch to two inches in thickness, and occur, one above another, in some places but a few inches, and in others several feet apart. These layers resist the action of the atmosphere for a great length of time, and only give away from the disintegration and wearing away of the rock beneath, when they break off and fall from their own weight. Between these layers the rock is sometimes very loose and friable, easily worked away with the pick.

It appears as if, during the deposition of this rock, that occasionally, in these localities, the surface was in some way covered with a sediment of the oxide of iron, which, acting as a cement, rendered this portion of the rock much harder and firmer than other parts of it. If you will examine one of these layers with a magnifying glass, you will see that they are made up principally of the same minute peculiarly formed grains of quartz, of which other portions of the rock is composed, stained and partially covered with the oxide of iron. We frequently find very beautiful ripple marks on these ferruginous layers. On some of them the impress of the eddies and ripples of the old Silurian ocean appear as fresh and palpable as if produced but yesterday. These markings are sometimes very singular and curious, mimicking the forms of organized life. Here is a specimen which I have been at a loss to determine whether it has been produced by the action of water or is an impression of some organized being. This rock is composed of small rounded grains of pure limpid quartz, which have a singular uniformity in their size and shape, in some places cohering so slightly as to crumble in the hand, and in other localities so firmly cemented as to make a good building stone. This rock is in some places of almost chalky whiteness, but more commonly it has a grayish as pect, while in other localities it has a reddish appearance, being stained with the oxide of iron.

As to the economical uses of this rock. There are several quarries on Franklin creek, in Lee county, and in Ogle county, where it has been pretty extensively used for building, and cut into window and door sills and caps. There was a beautiful arched bridge of cut stone, from one of these quarries, built over Franklin creek, for the Chicago and Fulton Railroad, when it was first constructed. Professor Hall says that this rock would make an excellent material for making glass.

It will be perceived that this rock, as it is found in the valley of Rock River, varies considerably from the description of it given by

Professor Hall, as it occurs in Iowa. Instead of its being uniformly the loose friable rock, spoken of by Mr. Hall, with scarcely cohesion enough to enable him to obtain cabinet specimens of it, we frequently find it forming bold perpendicular, and sometimes overhanging, cliffs, with strength and tenacity enough to make a good building stone. There are places where the rock is flinty and hard, and weathers out, like granite, in jagged and irregular peaks, high above the surface of the surrounding country.

BUFF LIMESTONE.

Next to the St. Peter's Sandstone, and separated from it in some places by two or three feet of shale and blueish clay, comes the Buff Limestone of Owen, classed by Hall with the Trenton Limestone. This is a thick bedded, compact, semi-crystalline magnesian Limestone, in layers of from one to two feet in thickness. It crops out in many places above the St. Peter's Sandstone. Between these thick ledges there are thin shaly layers, an inch or two in thickness, abounding in fossils. Although these layers are full of fossils, there appears to be but a very few species. They are very imperfect-most of them are casts, and appear to be such as are common to the Trenton Limestone proper. This rock is often quite fine-grained and compact, and makes an excellent building stone. From an analysis of specimens of this rock in Iowa, Professor Hall thinks that it may be very useful for the manufacture of hydraulic cement, as its composition was found to more nearly resemble than any of our other magnesian limestones, that of the best rocks used for that purpose in other places. These thick bedded layers are from twelve to eighteen feet in thickness.

TRENTON LIMESTONE.

The Blue Limestone of the Western Geologists, or the Trenton Limestone of the New York survey, succeeds these magnesian beds. This rock is quite variable in its appearance. In some places it has a blueish color, particularly on a recent fracture, but more frequently it is of a dull buff color. It is not so thick bedded as the preceding rock, and is in some places quite shaly, and breaks up into small fragments when quarried. In other places the layers are compact and thick enough to make a good building stone.

There are vertical crevices frequently found in this rock, which are from two to fifteen inches in width. Sometimes they are filled with debris, and in other places are open and serve as channels for subterranean streams of water from the pits in the elevated ground back from the bluffs, which I have spoken of above. At the base of the bluff, after a heavy shower, or at the breaking up of the winter, swollen streams of turbid water may be seen rushing from them.

The Trenton Limestone abounds in fossils. It is the oldest rock in this country in which we find a great profusion of the remains of organized beings, showing beyond doubt that the ocean of the lower Silurian era was filled with a multitude of the lower forms