Travels in North America, with Geological Observations on The

DXRW~ ON CORAL REEFS. 389 face level of the sea, in no one instance emerge above it. To escape this latter most improbable admission, which implies the existence of submarine chains of mountains of almost the same height extending over areas of many thousand square miles, there is but one alternative, namely, the prolonged subsidence of the foundations on which the atolls were primarily based, together with the upward growth of the reef-constructing corals. On this view every difficulty vanishes ; fringing reefs are thus converted in barrier reefs, and barrier reefs, when encircling islands, are thus converted into atolls the instant the last pin- nacle of land sinks beneath the surface of the ocean. " Finally, when the two great types of structure--the barrier reefs and atolls on the one hand, and fringing-reefs on tile other--are lald down in colours on the map, a magr, ificent and harmonious picture of the movements which the crust of the earth has within a late period undergone, is presented to us. We there see vast areas rising, with volcanic matter every now and then bursting forth through the vents or fissures with which they are traversed. We see other wide spaces slowly sinking without any volcanic outbursts, and we may feel sure that this sinking must have been immense in amount as well as in area, thus to have buried, over the broad face of the ocean, every one of those mom~tains above which atolls Pow stand, like monuments, marking the place of their former existence. Reflecting how powerful all agent with respect to de- nudation, and consequently to the nature and thickness of the deposits in accumulation, the sea must ever be when acting for prolonged periods on the land during either its slow emergence or subsidence: reflecting also on the final effects of these movements in the interchange of land and ocean-water on the climate of the earth, and on the distribution of organic beings, I may be permitted to hope, that the conclusions derived from the st.udy of coral formations, originally attempted merely to explain their peculiar forms, may be thought worthy of the attention of geologists." Pages 146-- 1't8. D. T. A. II. TRAVELS in NORTH AMERmA, with Geological Observations ou the UNITED STATES, CANADA, and NOVA SCOTIA. By CHARLES LYELL, Esq. F. R.S. 2 Vols. 12mo. pp. 588. Map and Plates. So far as regards geology, Mr. Lyell's work is eminently valu- able, since it presents a connected view of the results of a large number of careful surveys of different parts of the Continent of North America, by means of a coloured geological map, in which the whole amount of information at present known on the subject of North American geology is incorporated. The importance of this as a means of simplifying and general]sing the notions of English geologists with regard to the succession of strata on the other side of the Atlantic, it would be difficult to estimate too highly. Mr. Lyell has also added much to the knowledge hitherto possessed on the subject of American geology by his own investi- gations in the field, and in the present notice we propose to point out in order the various geological matters touched on in the work before us, commencing with the older rocks, and so ap- proaching last of all to those of newer date. The latter indeed, although perhaps the most important, we shall here scarcely allude to, because, having formed the subject of communications to the Geological Society, they either have been already or will be hereafter described in greater detail than even in the book itself cc3 890 NOTICES OF NEW BOOKS. in the Proceedings of the Society, as published in the various numbers of this Journal. The older Palaeozoic or Silurian rocks of America appear to be, as in England, divisible into two series--the upper and lower, and Mr. Lyell has admitted in his coloured map seven sub-divisions, which are thus named:-- {ii Hamilton group. Upper Silurian Helderberg series. Onondaga salt group. Niagara and Clinton group. Lower Silurian Trenton, &c. limestone group. Potsdam sandstone, &c. The Taconic system, named by Dr. Emmons from a chain of mountains which form a continuation of the green mountains of Vermont, and supposed to represent a group of formations more ancient than the Silurian, are not considered by Mr. Lyell as deserving an independent place among the rocks of the palaeozoic series (vol. i. p. 246.). (1) The Potsdam sandstones are chiefly developed in a narrow band on tile south-eastern range of the great chain, extending south-west from the Vermont range and along the whole of that line ; and they immediately succeed and overlie the granitic and gneissese rocks of Virginia, North Carolina, and Georgia. They are also found occupying a broader space on the banks of the great river St. Lawrence, between the north-eastern extremity of Lake Ontario and the city of Montreal; and here these most ancient of the fossiliferous rocks are loaded with the remains of .Lingula, and a small placunoid shell nearly allied to a fossil also occurring in company with the Lingula in the lowest English silurian beds at Builth in Brecknockshire (ii. 157.) Beds of the same Lower Silurian date have also been traced on the banks of the Wisconsin river, a tributary of the Mississipi, and Captain Bayfield is inclined to consider a band of sandstone on the southern coast of Lake Superior as the equivalent of these oldest fossiliferous sandstones of Potsdam. (2) The Trenton limestone is much more widely distributed in North America than the inferior beds of sandstone; but it appears difficult to separate it in some cases from the beds of the over- lying Hudson river group (3)* (ii. 49). The "blue limestone," as it is called, forming the hills and table lands around Cincinnati and elsewhere in Ohio, belongs to the upper part of the lower Silurian group, and abounds in organic remains, consisting chiefly of trilobites, brachiopodous shells, crinoidea, and many corals, the latter differing considerably in specific character from those in the The Hudson river group of Mr. LyelI ineludesa number of sandy and argillaceous slates containing Lower Silurian fossils, and separating in some eases the Trenton from the Niagara (Upper Silurian)limestones. LYELL'S TRAVELS IN NORTH AMERICA. 89i Lower Silurian strata of England (ii. 50.) With reference to the subject of Silurian fossils, Mr. LyeU takes occasion (i. 20., ii. 51.) to combat the notion that species were more cosmopolitan at this early period than they are at present. (4) The Niagara and Clinton groups form the base of the North American Upper Silurian series, and are not represented on the eastern flanks of the Alleghanies, the uppermost Upper Silurian bed being there in immediate contact with the Hudson river series. A vast tract of country on the west of this chain is, how- ever, occupied with a group consisting of these Niagara beds associated with (5) the Onondaga, and (6) the Helderberg series, but the sub-division is not here made out, although the different members are well seen in detail on the southern shore of Lake Ontario. The tIamilton group (7), the uppermost of the Upper Silurian series, is everywhere seen coming out from beneath the old red sandstone, which it seems to enclose as in a basin. The Niagara limestone and shale correspond in their fossils with the Wenlock and Dudley limestone of England, and overlie the Clinton group, which might almost be looked upon as lower Silurian, but which it is thought better in the present state of palmozoic geology to class with the upper members of the series. The Onondaga salt group (5)is a remarkable formation of red and green argillaceous shale, marl, and shaly limestone, sometimes of great thickness, but partially developed;and the Helderberg series (6), although consisting in the State of New York of a number of distinct beds, passes so insensibly into the lower group towards the west, that, as we have already observed, no well-defined line of distinction can be drawn. The Hamilton group (7) includes some shaly and slaty beds containing Ludlow fossils, and is widely distributed. It concludes the great Silurian series of North America, concerning which it may be remarked on the whole that with regard to the Alleghanies, the inferior or older beds range chiefly along the eastern or south- eastern flank, and are distinctly marked, while the newer groups of the same series, togegher with the Devonian and carboniferous formations, make their appearance as we proceed further west- ward. (ii. 9, 10.) The rocks of the Devonian period--the old red sandstone series of North Britain and Herefordshire -- are exhibited in a prominent and characteristic form, surrounding each of the great coal fields of the United States, and perhaps no where more strikingly than in the State of New York not far from Niagara. These beds here consist of olive-coloured slate and grey sandstone, containing occasionally impure coal, and in some sandstones (seen near Tioga) fragments of more than one species of Holoptychius have been obtained associated with a Chelonichthys of large dimensions. In Ohio, at the distance of 400 or 500 miles to the south-west, the author was struck by the extraordinary decrease in volume of the whole group, the absence of some formations, and the complete identity of those sets of strata that remained. In cc4 392 NOTICES OF NEW BOOKS. the sandstones of this period many ripple marks are found, and the surface of the slabs is frequently covered with fucoidal impres- sions; but fossils appear to be rare.

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