Niagara Limestone.-This is the third great mass of dolomite, which, throughout the valley of the Upper Mississippi, lies next above the Hudson river shales, and which, as well from the extent of surface covered by it as from its thickness and persistency of lithological and palæontological characters, forms one of the most important members of the Silurian series in the Northwest. It is one of the rocks which, prior to the recognition of the Hudson river shales in the Northwestern mineral region, was included under the designation of "Cliff limestone;" and, more recently, has been described as the "Coralline and Pentamerus beds of the Upper Magnesian limestone," the term "Upper Magnesian limestone," according to Dr. Owen, including all the members of the series from the base of the Galena, up to the base of the Hamilton group. The Niagara limestone of the region in question is a nearly pure dolomite having a crystalline structure and a light yellowish gray color; it differs but little in external appearance from the Galena limestone, and hand-specimens of the two rocks might frequently be mistaken for each other. The Niagara limestone, however, does not often exhibit that tendency to irregular decomposition, and consequent weathering in fantastic forms, so characteristic of the Galena; it also contains a greater amount of silica in the form of layers and nodules of flint, and it differs from this last mentioned rock also, in the fact that the fossils it contains are usually silicified, and not preserved in the form of casts simply. It may also be noticed that, whereas in the Lower Magnesian and Galena groups the amount of magnesia present is almost exactly that required to form with the lime and carbonic acid, the double carbonate, or dolomite; in the Niagara, on the other hand, there is frequently a small excess of lime over the magnesia. The thickness of this member of the series is estimated at about 350 feet, 250 feet being the greatest amount measured in any one exposure. The Niagara limestone throughout the Northwest is marked by the presence of beds crowded with the Pentamerus oblongus, as also by numerous corals, of which Halysites, Favosites, Heliolites, Syringopora and Lyellia are the most conspicuous genThere are also numerous Crinoids identical with, or closely allied to, those of the Niagara limestone of New York, but mostly in a very bad state of preservation, as might be expected in a crystalline dolomite.

era.

Le Claire Limestone. Of the groups recognized, in New York, as intermediate between the Niagara and the base of the Devonian, but a meagre representation has been observed in Iowa, and that only on the Mississippi river. The Le Claire limestone is described by Mr. Hall as a very heavy bed of dolomite, several hundred feet in thickness, which, in consequence of its disturbed condition and enduring character, has been the cause of the

Upper Rapids of the Mississippi. The fossils in it are all in the form of casts, and among them are, a small Spirifer, a Spirigera, a Pentamerus, undistinguishable from P. occidentalis, several Gasteropods and some chambered shells. This dolomitic mass is placed, conjecturally, on a parallel with the Galt limestone of Upper Canada, hitherto supposed to form the base of the Onondaga Salt Group. The Le Claire limestone has not been traced to any distance from the river, and is certainly wanting in northeastern Iowa, where the Niagara limestone is overlaid directly by rocks of the Hamilton group. It appears that a further examination should be made of the section at the Upper Rapids, as it is difficult to understand how so thick a mass of rock should appear and disappear without having been recognized anywhere except at that one point.

The Onondaga Salt Group, a member of the Silurian series of so much economical importance in New York, is represented in the Mississippi river section by a few feet only of magnesian limestone, or nearly pure dolomite, although soft and destitute of crystalline structure. The peculiar physical condition of the Onondaga Salt Group, as it exists in New York, is exhibited in some of the layers of the section on Quarry creek, a small tributary of the Mississippi, but the economically valuable minerals are wanting. This group has not been traced west of the Mississippi, where, indeed, it exists only in a few detached frag

ments.

Of the rocks of Devonian age, the Upper Helderberg Limestone, so well marked in New York and Ohio, is with difficulty to be recognized to the west of the Mississippi. Certain non-fossiliferous strata cropping out on the bank of that river, at, and for two or three miles above, Davenport, are referred by Mr. Hall to that group, chiefly, as it appears, from their stratigraphical position and lithological character.

The Hamilton group is an important member of the series in Iowa, covering many hundred square miles of surface, although greatly diminished in thickness from what it was in New York. It consists of a series of purely calcareous and calcareo-magnesian strata, with occasional bands in which argillaceous matter occurs to some extent, the lithological character of this portion of the series being somewhat more variable than that of the groups below. The greatest thickness of this group exposed in any one section appears to be about one hundred feet; but its entire development has not been satisfactorily ascertained. A few of the species of fossils found in it are identical with those occuring in New York; but most of them are new. Several are more like Devonian species of Central Europe than any which had been previously described from this continent.

The Chemung group in the Mississippi valley gives but a meagre representation of the same series of rocks in New York, Pennsylvania and Ohio. It contains almost an entirely new Fauna, although of species closely allied to those of more easterly localities of the same group.

The passage from the Devonian to the Carboniferous series is shown, in this Report, to be an almost imperceptible one, both in the physical and paleontological character of the groups, there being no strong line of demarcation separating the upper calcareous beds of the Chemung group from the Burlington limestone, the lowest member of the Carboniferous limestone series.

One of the most interesting facts brought out in this connection, is the existence of five distinct members of the Carbonif erous limestone series. These are shown to have been deposited in an ocean which was gradually contracting its limits on the north, the greatest development of each successive member of the series, in an ascending order, being to the south of the one below it; while, subsequent to the deposition of all these and the sandstone which separates the fourth and fifth limestones, the entire area was submerged, allowing the coal-measures to be deposited on the slightly inclined edges of all these limestones, as well as of the Chemung and Hamilton rocks, and also, to some extent of the Silurian limestone, after they had been disturbed and denuded.* All these limestones of the carboniferous series are well characterized by the fossils they contain.

The fossils of the survey are described and figured in Part II on Palæontology, by Prof. Hall, this portion of the volume extending to 250 pages. The Devonian and Carboniferous series have been selected for illustration, as the Silurian had previously received much attention in the Reports of Dr. Owen. In the Hamilton and Chemung groups the fossils are particularly interesting, as exhibiting the influence of geographical conditions, or of distance, while the physical condition remained nearly the same as in central and western New York. The entire number of species described from all these rocks is about

250.

Special attention is given in the Report to the fossils of the Carboniferous limestones, as illustrating the successive members of the series; and with this object contrasting forms have not been selected, but, on the contrary, the more common and characteristic fossils of each rock. Many hitherto believed to be identical with European species are proved to be quite distinct. The number of species of Crinoids, described in the volume, is probably equal to or greater than all those before made known * See this Journal, [2], xxiii, 187. SECOND SERIES, VOL. XXVII, No. 79.-JAN. 1859.

from the same formations. Those of the Carboniferous limestones amount to one hundred and seventeen species, and of these eighty-nine are new or not before described.

The true generic characters of Zeacrinus, Agaricocrinus and Agassizocrinus of Troost,-genera which that author had given in his catalogue, but of which he had never published the full descriptions are here for the first time given. In this and other cases Mr. Hall has evidently aimed to recognise fully the unpublished labors of Prof. Troost. He has on page 544 the following note:

"I have transcribed these observations, as well as the specific description of this species, from the MS. of Dr. TROOST's memoir upon the Crinoidea, which is to be published in the Smithsonian Contributions to Knowledge; having been permitted to make such references and citations as would enable me to verify any of the genera and species which I might describe in the Iowa Report. By this means, although Dr. TROOST's paper has not yet been published, he has the precedence which belongs to him."

The Scaphiocrinus in its typical species so nearly resembles the Graphiocrinus of De Koninck and Le Hon that Mr. Hall is led to suspect that they have overlooked a series of small basal plates. These characters are here illustrated; and in the Forbesiocrinus is shown a series of three basal plates below the five plates supposed by these authors to constitute the base. This genus is further sustained by five American species, all of which are new.

Some interesting facts are shown, for the first time, connected with the structure of the Actinocrinus, and particularly the distribution of the arms and their relations to the rays or radial series of the plates of the body. These relations, as well as other important points for the discrimination of species are shown in the diagrams accompanying many of the descriptions with a formula of numbers belonging respectively to the anterior, anterolateral and postero-lateral rays. These characters, shown to be constant, offer important additional facilities for the determination of species, especially where specimens are imperfect.

Of the genus Actinocrinus alone twenty-nine new species and two varieties are described, and of the genus Platycrinus sixteen new species. Rhodocrinus is noticed for the first time among American Carboniferous species. Five species of the genus Archæocidaris are described and illustrated, one from each of the limestones of the Carboniferous series.

This Report is doubtless the best contribution yet made to our knowledge of the Crinoids and other Echinoderms of the Carboniferous system; and, both as regards their structure and their geological distribution, it is of the highest interest. We might cite at length important observations on the genera and species,

did space allow. The plates are all good, and those of Crinoids remarkably beautiful and effective.

In the chapter devoted to Economical Geology, we find a large number of analyses of the rocks, coals, and other materials of economical value occurring in Eastern Iowa. The limestones analyzed are many of them almost chemically pure dolomites; and, in general, the predominance of crystalline carbonates of lime and magnesia over the purely detrital rocks is very marked all through the series. There is a striking deficiency of the argillaceous element, especially, in all the Silurian rocks. The analyses, taken together, exhibit a tendency, as we rise in the geological scale, to a greater variety of lithological character in the members of the successive groups, a greater amount of detrital matter and a diminution in the quantity of magnesia, there being no heavy and persistent bed of dolomite above the Silurian.

Among all the specimens examined, the only ones found to contain a sufficient quantity of insoluble matter to be available for hydraulic lime are those from the Buff limestone, at the base of the Trenton. It remains to be ascertained, by practical trials, how far the dolomites and highly magnesian limestones, with but a small quantity of insoluble matter, can be used for hydraulic purposes, as they have been to a limited extent in Virginia and also in France.

The analyses of numerous samples of coal show that they belong, like all the Western coals, to the highly bituminous class; they contain from 35 to 40 per cent of bituminous matter, and from 45 to 50 of fixed carbon. They all appear to hold a large amount of hygrometric moisture with great tenacity, parting with it slowly, and not until after years of exposure to a dry atmosphere. Some samples give as much as fifteen per cent of water, expelled by drying at a temperature of 212° F. Sulphur is present in all these coals, in a form not perceptible to the eye, to the amount of from one-half of one to two per cent, and also in much larger quantity, in combination with iron and lime, as pyrites and gypsum, which substances materially impair the value of western coals. No workable iron-ore of any importance has been discovered in connection with the coal-measures of Iowa, which are exceedingly thin, no section having been measured in the Des Moines valley giving much over a hundred feet

in thickness.

The subject of the occurrence of the lead ore in the Lower Silurian limestones of the Upper Mississippi valley, and, especially, within the limits of Iowa, is discussed in this Report on

pages 422 to 468. The principal crevices, or lead-bearing fissures, which have been worked in the vicinity of Dubuque are described, and a diagram given illustrating their surface