be termed beds of passage to the Utica slate, and have not, to the writer's knowledge, been found lower in the formation. During eight years of work in the Trenton limestone I have never seen a fragment of this trilobite below the upper shaly passage beds. It has a distinct geological horizon, and its presence indicates a parallelism of formations as will be shown by the facts presented in this paper. The presence of the Utica slate horizon does not rest solely on the presence of this trilobite as there is both a palæontological and lithological break in the series, at this horizon, throughout the Appalachian basin with the exception it may be at the extreme north-western and south-eastern outcrops.

In South-western Illinois, the formation resting on the Trenton limestone is described by Professor A. H. Worthern (Geol. of IU., III, p. 27, 1868), as a sandstone, which he calls the "Thebes sandstone and shale." He says:

"This formation, which underlies the limestone above described (dark blue compact limestone) is well exposed in the vicinity of Thebes. *****The lower portion of it only is a true sandstone, and is about thirty feet in thickness, and passes upward into a sandy shale of a dark brown color. ***** A half mile below Thebes we found a yellowish brown shale, apparently not above five or six feet in thickness, that evidently formed the base of this group. It was filled with fragments of Trilobites, apparently belonging to the Asaphus canalis, which, with a Lingula found in the upper shale immediately below the limestone, are the only fossils it has afforded. ***** Some of the sandstone layers are from two to three feet in thickness and well adapted for building purposes."

In the valleys of Virginia, Eastern Tennessee, and Northern Alabama, the black carbonaceous shales of the Utica slate are replaced by the lighter colored marls and shales of the same geological horizon.

Professor James Safford in the Geology of Tennessee, pp. 228273, 1869, describes the Trenton and the Nashville rocks in East Tennessee as follows:

"They are, first, a stratum of blue limestone, more or less argillaceous, from 200 to 600 feet thick, then, above this, a great body of sky-blue calcareous and often sandy shales. **** In going to the north-west the shales become more and more calcareous approaching the condition of the same strata present in Middle Tennessee.

"At the base of the sky-blue calcareous shales in the eastern border counties there is a fine dark or black shale, becoming in places 100 or 150 feet thick. It abounds, very generally, in graptolites. The graptolites are not confined to this lower stratum, they run up into the main body of the shale and are found at numerous localities.

"In the basin of Central Tennessee the rocks are mainly blue limestones throughout. The entire series is about 1000 feet thick

and is equally divided between the Trenton and Nashville series. This series is a natural group, and though each member has many of its own, yet there are a number of forms uniting the two. It is divided both on lithological and paleontological grounds into two sub-groups. The Trenton ends with a light colored, heavy-bedded limestone, and the Nashville begins with a silicious, blue calcareous rock, weathering often, into thin, earthy, buff, sandy masses, and, sometimes, into shales. The bed is distinguished by the abundanee of Orthis testudinaria and Strophonema alternata, while the upper Trenton beds contain Stromatopora rugosa, Columnaria alveolata, Petraria profunda, Strophonema filitexta, Rhynchonella recurvirostra, Pleurotomaria lapicida, Orthoceras Huronense, etc.

"The Orthis bed is frequently a group of smoothly laminated flags, interstratified with shaly seams, when wet it looks much like the 'Black Shale.' At almost all points where it comes to the surface throughout middle Tennessee, it is seen to contain vast numbers of individuals of Orthis testudinaria.

"In the western valley of Tennessee it is the hydraulic rock and the most conspicuous of the Nashville strata. Along the Tennessee river it is seen at the base of several bluffs, its dark band on a level with the water and in strong contrast with the lighter beds, and gray limestones of the high bluffs above.

"The upper members of the Nashville group, constitute a group of rather dark blue, highly fossiliferous, often roughly bedded, impure, limestones of a maximum thickness of 400 feet."

The preceding descriptions, selected from the various authors who have published details of the character and sequence of the strata of the area and geological horizon over which our examinations have taken us, proving the existence of a widespread change in the physical condition of the sea of the Appalachian basin at the close of the Trenton limestone formation, and preceding that of the coarser and more varied sediments of the Hudson River formation, are given as a basis for including the Utica shale, Galena limestone, Triarthrus beds of Cincinnati, and the Orthis bed of the Nashville series, in one geological epoch; we might also add the Thebes sandstone and a portion of the Graptolitic shales of Virginia, Tennessee and Alabama.

All along the margins of the great Appalachian basin, from Anticosti to Lake Huron, to Minnesota, to Alabama and thence northeast to the St. Lawrence, the change at the close of the Trenton limestone-forming period, in the lithological characters and the fauna, is everywhere apparent; in some localities quite marked and abrupt; in others the horizon is nearly lost by the blending of both the strata and organic remains of the lower with the next succeeding formation. The organic remains of the Utica slate entitle it to rank as a formation quite as much as its lithological character.

one

hundred species known to the writer as occurring therein, fifty-four are peculiar to it and not known above or below its horizon. The graptolites are the most constant forms but over a great area the Triarthrus Becki is only second to them. The few individuals of the species of trilobite that have been found in the Trenton limestone were in the upper shaly limestone, which was deposited when the conditions bringing about the Utica slate deposit were being introduced.

Triarthrus Becki and the various forms of graptolites appear to have been, notwithstanding their fragile character, peculiarly adapted to spread over an extended area in the muddy bed of the sea, while the clearer limestone-forming seas were not favorable to the development of the trilobite and to but few species of the graptolites ; the trilobite has not been found to the knowledge of the writer to the north-west, west, or south of the Cincinnati exposure. That this trilobite and the graptolites should have obtained so wide a geographical distribution is evidence of the comparative slow deposition of the sediments forming the Utica slate; this view is also strengthened by the presence of the large and fully developed Asaphus Halli and Asaphus Canadensis at Collingwood, where the shales are filled with their remains and those of graptolites, brachiopods and orthoceratites.

The return of conditions favorable to the existence and extension of the graptolites and Triarthrus Becki, at the commencement of the deposition of the Hudson River formation in Illinois, Wisconsin and Iowa, would have undoubtedly led to their extension to the west, either by the way of the central basin or the northern coast line, had they not been replaced by the fauna of the later beds at Cincinnati. I would regard the deposition of the sediment forming the Galena limestone as going on during and after the deposition of the Triarthrus beds at Cincinnati; the calcareous marls and sedi ments of the Hudson River formation over the central basin, to the west of the extension of the Utica formation, gradually forcing their way, towards the close of the epoch, over the Galena limestoneforming area, and eventually burying it beneath a deposit of shales. and preventing for a time the deposition of the sediments forming the magnesian limestones, so characteristic of the western side of the Appalachian basin.

The shales resting on the Galena limestone in Wisconsin and Iowa, cannot, in the writer's view, be considered as the possible or probable equivalent of the Utica slate. There is no similarity in the fauna and the fact of their being somewhat carbonaceous is not sufficient to correlate them.

The Galena limestone in its north-eastern extension commences as a comparatively thin formation and augments in volume to the west and south-west. In the north and north-western exposures, it is essentially a connecting link or bed of passage between the Trenton and Hudson formations. Distinct, lithologically and palæontologically, when viewed by itself, as is the Utica slate, still, on the same grounds, it is united to the formations above and below. To the underlying Trenton it is connected by beds of passage and the presence of fiftysix species of fossils found in the Trenton, twenty-nine of which do not pass upward into the Hudson River formation. In the northeastern portion it partakes of the character, in its upper beds, of the succeeding beds, and thirty species continue on into the Hudson River formation, while nineteen species of fossils are limited to the Galena.

The following table gives the number of species in each of the two formations, viz.: Utica and Galena.

Number of species limited to the formation and Trenton group, Number of species limited to the formation and the Hudson River formation,

Number of species common to the Trenton, Hudson River, and U. and G. formations,...

Number of species passing from the Trenton formation to Utica and Galena,.

Number of species passing from the Utica or Galena to the Hudson River formation,...

The table shows that a greater change took place in the fauna of the Utica slate than in that of the Galena limestone; the former having fifty-four species limited to its boundaries, and thirty-six derived from the Trenton; while the latter has nineteen species peculiar to it, and fifty-six passing up from the Trenton formation beneath. This diversity is undoubtedly owing to the greater variation in the character of the sediments of the Utica slate as compared with the Galena, when the change from the Trenton limestone-forming deposit occurred. One of the strongest arguments in favor of the Galena being placed with the Utica slate in a distinct epoch is the fact that, notwithstanding its being a limestone formation, it has a fauna of twenty-two species, which do not occur in the Trenton and but three pass to the Hudson River formations. If the Utica slate

This does not include the undefined species from the Galena, mentioned in the Geology of Wisconsin, II, 1877.

can be taken from the Hudson River group and considered a distinct formation then the Galena is to be ranked in the same way, both on account of its lithological characters and the peculiarity of its fauna. To the superior formation the Galena is united by passage beds and thirty species of fossils, while the Utica slate has but two more species and a somewhat closer union by its lithological characters. Both the Thebes sandstone and the Orthis bed of Tennessee give expression to the epoch following that of the Trenton and, although inferior in volume and character of imbedded fossils to the Utica and Galena, they unite with the Triarthrus beds of Cincinnati to establish the Utica horizon over the central Appalachian basin, at the close of the Trenton limestone-forming epoch.

In conclusion, it is to be considered, that, although an equivalency in time cannot be asserted for the deposition of the Utica and Galena formations, the facts presented afford strong evidence of their belonging to the same geological epoch, and considered with the beds mentioned above in the central basin, they form a distinct geological horizon in the Trenton period, separating and distinguishing the two great divisions, the Trenton and Hudson River.

The Utica horizon will then include:

The Utica slate.

The Galena limestone.

The Triarthrus beds of Cincinnati.

The Orthis bed of Tennessee.

The Thebes sandstone.

Graptolitic shales, in part, of Virginia, Tennessee and Alabama.

We have observed that there is a well defined and distinct horizon between the Trenton and Hudson River formations along the Hudson and Mohawk valleys, which extends throughout the Appalachian basin. With this in view there can be but one series of terms to express the divisions of Prof. Dana's Trenton period.

They are:

HUDSON EPOCH.
UTICA EPOCH.
TRENTON EPOCH.

The term Cincinnati epoch cannot be substituted for that of Hudson River, without violence to accepted rules of nomenclature. The term Nashville group has priority and both are synonymous with the much older term Hudson River. The Hudson epoch, as used above, includes the original Hudson River group of the