CHAP. XVI.

GEOLOGY OF OHIO.

39

CHAPTER XVI.

Succession of Strata on the Ohio between Pomeroy and Cincinnati.-Return up the Ohio to Rockville.-Waverley Sandstone.-Cliff Limestone.-Denudation.-Blue Limestone of Cincinnati.-Lower Silurian Fossils.-Limited Proportion of Silurian Species common to Europe and America.-Great Development of Brachiopoda.-Deep Sea Formations.—Rarity of Silurian Land Plants.-Silurian Fossil Fish.

BETWEEN the western extremity of the Appalachian coal field and Cincinnati, as will be seen by referring to the section given at p. 92, Vol. I., the different formations, from the Devonian to the Lower Silurian inclusive, come up to the surface in succession, being well exposed in the cliffs or steep slopes of the hills which bound the Ohio valley. I have already mentioned a fine seam of coal at Pomeroy, beneath which, farther to the westward, the lower coalmeasures are seen, and at length at Portsmouth, the inferior conglomerate or millstone grit, next to which, the formation, called by the Ohio geologists the Waverley sandstone, the equivalent of the Devonian formation (No. 9, in the large map, Pl. II.) makes its appearance. To this sandstone, the Upper Silurian slates and limestones (Nos. 10, 11, and 13 of the map) succeed in the descending order, and lastly, at Cincinnati, the Lower Silurian groups (Nos. 14 and 15 of the map) are exhibited in the hills, and in the bed of the Ohio at low water.

Having, when I came down the Ohio, made the

40

GEOLOGY OF OHIO.

CHAP. XVI.

last part of my journey in the dark, I re-ascended the river for a hundred miles, in company with Dr. Locke, a geologist lately engaged in the State survey of Ohio, and who liberally devoted his time to aid me in my inquiries. I was desirous of seeing the rocks corresponding to the Old Red sandstone before mentioned, and with this view we landed at Rockville, about eighteen miles below Portsmouth, and examined the Waverley sandstone at that place. Retaining in my mind a perfect recollection of the aspect of the deposits intervening in the State of New York, between the Coal and the Upper Silurian groups, at the distances of 400 and 500 miles, I was struck with their extraordinary decrease in volume, the absence of some formations, and the complete identity of those sets of strata which remained. I have before alluded to the gradual thinning out of the coarse sedimentary rocks, both in the Silurian and Carboniferous series of the U. S., as we proceed westward, and the increased thickness of many of the calcareous formations. The Waverley sandstone of Rockville has been recognized by Mr. Hall as the representative of the Chemung and Portage groups of the New York Reports. It contains here many ripple-marked flags with partings of shale. The surfaces of the slabs of sandstone display the festoon-shaped fucoid, called here Fucoides cauda galli, from its resemblance to a cock's tail. I saw some single individuals of this plant extending through layers eight inches thick. There were no associated shells; but in some of the uppermost strata of the series we found spirifers and other brachiopods, with many encrinites.

Between Rockville and Cincinnati the bituminous shales corresponding to the Hamilton group (or No. 10 of the large map) are seen, and below them what is called the "Cliff limestone," which is considered, and, I believe correctly, by Mr. Hall, as the representative both of the Helderberg and Niagara limestones of New York. Among the characteristic shells, I observed the Pentamerus oblongus, so abundant in the Clinton group of New York, a shell considered by Messrs. Murchison and De Verneuil to mark the line of separation between the Upper and Lower Silurian rocks of Europe.

In discussing with Dr. Locke the probability of the former continuity of the Illinois and Appalachian coal-fields (see the section, Vol. I. page 92), and the possible extension of the strata (Nos. 5, 6, and part of 7 of that section, or 9, 10, 11, and 13 of the large map) over that flat dome on the middle part of which Cincinnati is built, we endeavoured to calculate the height which the central area would have attained, if the formations supposed to have been removed by denudation were again restored. In that case the thickness of the strata of coal, subjacent conglomerate, Devonian and Upper Silurian beds, which must have been carried away, could not, if we estimate their development from the mean of their aggregate dimensions on the east and west of Cincinnati, have been less than 2000 feet. The tops of the hills near Cincinnati, composed of the blue limestone, are about 1400 feet above the level of the sea. If, then, the formations presumed to have been destroyed by denudation were replaced, the height of the dome

42

BLUE LIMESTONE.

CHAP. XVI.

would be about 3500 feet, or exceeding the average elevation of the Alleghany Mountains.

The thinning out and disappearance of the mudstones and sandstones of the more eastern States, causing limestones, such as the Helderberg and Niagara, so widely separated in New York, to unite and form single and indivisible masses in Ohio, affords no argument against the classification of the New York geologists. Their grouping of the subordinate members of the Devonian and Silurian systems has been based on sound principles; on mixed geographical, lithological, and paleontological considerations; and the analogy of European geology teaches us that minor subdivisions, however useful and important within certain limits, are never applicable to countries extremely remote from each other, or to areas of indefinite extent.

The rock forming the hills and table lands around Cincinnati, called the blue limestone, has been commonly referred to the age of the Trenton limestone of New York (No. 15, map, Pl. II.), but is considered by Messrs. Conrad and Hall, and I believe with good reason, as comprehending also the Hudson River group (No. 14 of map). It seems impossible, however, to separate these divisions in Ohio, so that the district coloured blue (No. 15) may be regarded as agreeing with Nos. 14 and 15 in other parts of my map. Several of the fossils which I collected at Cincinnati, the encrinites and Aviculæ (of the subgenus Pterinea) in particular, agree with those which I afterwards procured near Toronto, on the northern shores of Lake Ontario.

After seeing at Cincinnati several fine collections

of recent and fossil shells in the cabinets of Messrs. Buchanan, Anthony, and Clark, I examined with care. the quarries of blue limestone and marl in the suburbs. The organic remains here are remarkably well preserved for so ancient a rock, especially those occurring in a compact argillaceous blue limestone, not unlike the lias of Europe. Its deposition appears to have gone on very tranquilly, as the Lingula has been met with in its natural and erect position, as if enclosed in mud when alive, or still standing on its peduncle. Crustaceans of the genus Trinucleus are found spread out in great numbers on layers of the solid marl, as also another kind of trilobite, called Paradoxides, equally characteristic of the Lower Silurian system of Europe. The large Isotelus gigas, three or four inches long, a form represented, in the Lower Silurian of Northern Europe, by the Asaphi with eight abdominal articulations, deserves also to be mentioned, and a species of graptolite. I obtained also Spirifer lynx in great abundance, a shell which Messrs. Murchison and De Verneuil regard as very characteristic of the lowest Silurian beds of Russia and Sweden. Among the mollusca, I may also mention Leptæna sericea, Orthis striatula, Bellerophon bilobatus, Aveculæ of the subgenus Pterinea, Cypricardia, Orthoceras, and others. There were also some beautiful forms of Crinoidea, or stone-lilies, and many corals, which Mr. Lonsdale informs me differ considerably from those hitherto known in Britain, a circumstance probably arising from the small development of coralline limestones in the Lower Silurian strata of our island. Several species