a comparison of these some light is thrown on another question which has been discussed without having been satisfactorily answered, namely: What is the difference between the climate of the Eastern and Western States on the same parallel? It appears that while there is but little difference in the mean temperature of the year on the Atlantic coast and in the Mississippi valley, in the same latitude, there is a perceptible tendency to extremes in the mean of the seasons, the summers being hotter and the winters colder, as we go farther west. The annexed table (from p. 33) will serve to illustrate this statement.

In the geological portion of the Report, we notice the following matters as of more especial interest.

The members of the geological series developed in Eastern Iowa, all belong to the Paleozoic system, and include groups of strata from the Potsdam Sandstone up to the coal measures. The existence of the Permian in the central portion of the state is inferred from the presence of large masses of gypsum overlying the coal measures, but in connection with which no fossils have as yet been discovered. The range and extent of the formations is exhibited on the geological map accompanying the Report, a glance at which will be more satisfactory than an attempt at description. It may simply be noted that the general trend of the formation is northwest and southeast, and that the dip being to the south and west, the traveller, in passing over the State from northeast to southwest, crosses successively higher groups. There appears to have been but little disturbance of the strata since their deposition, and no igneous or metamorphic rocks are known to exist within the limits of the State.

The Silurian series, as it is developed in the Northwest, is made up of alternations of sandstones, dolomites, limestones, and shells. The order of succession, lithological character and thickness of the different members which are recognized in Iowa, may be seen in the annexed table, arranged in an ascending order:

In general there is shown a thinning of the members above the calciferous sandstone as compared with the series in New York, and a disappearance of some of them. A few particulars with regard to the various groups are here added.

Potsdam Sandstone. The very great thickness of this formation, which has been shown to exist in the Lake Superior region, is limited to the vicinity of the trappean rocks. Proceeding southwesterly from the copper-bearing range, we soon find the conglomerate to have disappeared, and have no evidence in Iowa and southern Wisconsin of the existence of more than 400 feet at any one point, while the mean development is probably not over 250 or 300 feet. The exposures of this rock are very limited in Iowa, but it covers considerable surface in Minnesota, and still more in central Wisconsin. Bands or intercalated masses of conglomerate are almost entirely wanting in the sandstone; these, as well as the lines of oblique lamination, appear to be confined to the vicinity of the igneous rocks. There is no member of the series more persistent, both in lithological and paleontological characters in the lower sandstone; it has been traced from lon. 73° to lon. 104°, exhibiting everywhere the same granular silicious character and characterized by the same organic forms, and we have, up to this time, no evidence of the existence of organic remains below this formation.

Lower Magnesian Limestone.-This is a mass of dolomite, hav ing a thickness of from 225 to 250 feet, about 200 of which are nearly pure, crystalline dolomite, containing from one to ten per cent. of silicious sand, mechanically intermixed; the remaining 25-50 feet are beds of passage into the sandstone below, consisting of mingled and alternating sandstone and dolomite. Fossils are extremely rare in this member of the series; a few have been observed in Wisconsin in a very imperfect state of preservation, but none in Iowa.

Upper or St. Peter's Sandstone.-This repetition of the sandstone underlying the lower magnesian, is also remarkable for its persistence in lithological character and thickness over a great extent of surface. From La Salle, in Illinois, where it makes its appearance in a low axis of elevation, underlying the coal-measures unconformably, to St. Paul in Minnesota, a distance of over 400 miles, this sandstone hardly varies more than ten feet from its normal thickness of about 80 feet, which indicates a remark

able uniformity in the physical conditions prevailing at the time of deposition of this comparatively thin mass. The fact that this sandstone is so persistent in its thickness and lithological character; that it consists of almost chemically pure quartz, in the form of grains of minute, but uniform size, with crystalline facets; that it contains no pebbles or fragments which can be recognized unmistakably as being of foreign or detrital origin, are noticed as giving plausibility to the supposition that it was a chemical precipitate, rather than the result of the mechanical disaggregation of pre-existing quartzose rocks. No fossils have been found in this sandstone.

Trenton Limestone.-Under this head is designated the series of beds between the Upper Sandstone and the Galena limestone, which may be subdivided into two portions: a, the buff limestone, an impure dolomite, containing from ten to twenty per cent of sand and clay; it is from fifteen to twenty feet thick, and is, in the vicinity of the Mississippi river, quite destitute of fossils. It is succeeded, in the ascending order, by b, the blue or Trenton limestone proper, a series of calcareous and calcareoargillaceous layers, lime unaccompanied by magnesia appearing here for the first time in the series, the whole having a thickness of from 70 to 80 feet. For the first time, also, we find traces of organic life abundantly disseminated through the rocks, a fact not without significance in its relations to the absence of magnesia noticed above. Many of the layers, and shaly partings between the compact calcareous beds, are crowded with forms either identical with, or closely allied to, those which characterize the Trenton limestone in its extension from New York through Canada and on the northern shores of Lakes Huron and Michigan, and as far west as the Mississippi, along a line of outcrop some 1500 miles in extent.

Galena Limestone. The passage from the Trenton limestone into the next succeeding member of the series, the Galena limestone, is not an abrupt one; on the contrary, there are, in many localities, several alternations of calcarco-magnesian and purely calcareous layers, indicating that the change of conditions which resulted in the deposition of the highly crystalline dolomite which overlies the Trenton was not effected at once, nor without occasional partial returns to the former state of things. The Galena limestone, as usually developed, is a rather thick-bedded, light greyish, or light yellowish grey, dolomite, distinctly crystalline in its texture and usually rather coarse-grained. The more crystalline portions frequently contain cavities lined with small crystals of brown spar, and the rock is remarkable for the irregularity with which it weathers, leaving picturesque outliers, with castellated forms, like watch-towers, or the half-ruined walls of ancient fortified cities. The quantity of insoluble matter in

this dolomite is very small, not usually exceeding two or three per cent; it consists almost entirely of quartzose sand. The fossils of the Galena limestone are closely allied to those of the Trenton, although, in the lead-region, certain ones, such as the Receptaculites and Lingula quadrata, which are characteristic of the Galena limestone, are not found in the underlying blue. The fossils of the Galena are all in the form of casts with the exception of those in which the shell originally consisted of phosphate of lime instead of the carbonate, as was the case with the Lingula; it appears, therefore, that the chemical changes which the rock has undergone since its deposition have been such as to remove the substance of shells consisting of the carbonate of lime, but to leave the phosphate untouched. Another interesting fact in connection with the paleontology of the Galena limestone is, the discovery in it of a single specimen of Halysites catenulatus, the characteristic and most abundant fossil of the Niagara limestone in this region. This coral was in the form of a cast, while those of the Niagara are uniformly silicified, this, with other circumstances, removing all possibility of error in regard to its true locality. A single specimen of the same genus is described by Mr. Hall as occurring in the Hudson river group on Green Bay, these two being the only instances in which this genus has been found in the United States in Lower Silurian rocks.

The Galena limestone forms a very important member of the series in the Upper Mississippi valley and in Wisconsin, although not distinctly recognized to the eastward of the Menomonee river; it is fully 250 feet thick in the vicinity of Dubuque, where it has its maximum development, and from which point it gradually thins out in every direction. It is, economically, of high interest, from the fact of its being the chief repository of the lead ore which has been, and still is, so extensively mined in the Upper Mississippi lead-region.

Hudson River Group.-This member of the series according to the Report is first distinctly recognized in its extension west of Little Bay des Noquets. Being composed chiefly of silicious and silico-argillaceous shales, which disintegrate with rapidity, so that a good natural section is rarely exposed, its existence in the Upper Mississippi valley was for a long time overlooked, although many of the shafts in the lead region are sunk through a greater or less thickness of it, in order to reach the underlying limestone. It did not escape the observing eye of Percival, who mentions it in his first annual Report, dated 1855, under the name of "Blue Shale," but without any indication of its paleontological relation or thickness, as, indeed, it has, within the limits of the lead-region in Wisconsin, been almost entirely removed by denundation. The thickness of these shales, when fully developed is from 60 to 80 feet, and in some places perhaps, as much as

100, but no natural section has been observed exposing more than 25. The quantity of organic remains crowded into some of the layers of this group, is truly astonishing; some strata of six or eight inches in thickness are made up of Orthocerata packed as closely together as they can lie. The paleontology of this formation has not been investigated in detail, but its position and the general character of its fossils leave no doubt of its equivalency with the Hudson river group.

An interesting fact in connection with these shales, is the large amount of bituminous matter which they contain, and which is shown in this Report to be characteristic of this group from New York, through Canada, to the Mississippi river. A specimen of a dark chocolate-colored shale from Savannah, Ill., was found to contain 20-96 per cent of combustible substances: other specimens from the vicinity of Dubuque, lost, on ignition, from 11 to 16 per cent. of organic matter. The black, highlyglazed and apparently very carbonaceous shales of the Hudson river valley, which have been so frequently mistaken for coal, contain from one-half to one per cent of carbon, but no volatile matter; while specimens of the Utica shale from Herkimer Co., on the other hand, lost from 12 to 14 per cent of their weight when burned in oxygen.

The presence of carbon in the shales of the Hudson river group over so extensive a region, and in so large quantity, is not only a matter of very considerable economical importance, as indicating a source from which, in those parts of the country where the true Carboniferous rocks are wanting, a supply of material for lighting, and perhaps heating, purposes may be obtained; but it is also of great interest in a theoretical point of view, as bearing on the question of the origin of the carbon in the coal-measures themselves. These shales and slates seem to have been accumulated under conditions somewhat resembling those which prevailed during the deposition of the Carboniferous series, while the presence of so large a per-centage of carbon in them is rendered still more striking by the fact, that, in the Northwest, neither the rocks below, nor those above as far up as the coal-measures, contain more than the merest trace of carbonaceous matter. From the base of the Potsdam to the top of the Galena limestone, the whole amount of carbon present in the rocks, would not, if collected into one layer, make a deposit of more than an inch or two in thickness; but if the bituminous matter of the Hudson river shales at Savannah, were all collected by itself in one stratum, instead of being diffused through perhaps 60 or 80 feet of shale, that stratum would have perhaps equalled twenty feet or more in thickness. A further investigation into the exact nature and distribution of the bituminous matter is contemplated.