nearly parallel to that of Wenlock Edge.

stones' of the upper Silurian rocks, this formation is composed essentially of sandstones of different colours, with occasional subordinate courses of calcareous matter, especially at the upper part. Its best and clearest distinctions, however, consist in its infraposition to the upper Silurian rocks; and its organic remains, nearly all of which are dissimilar to the fossils of the 'formations which immediately overlie it.'-(P. 216-222.)

A dark purplish-red sandstone occurs in the bed of the Onny, amidst the Caradoc strata, in nearly vertical strata much distorted; which is unlike any of the upper beds, and very much resembles part of the old, or even of the new-red sandstone. Some of the other beds, at the same place, might almost be mistaken for sandy claystone, of trappean origin; and it will be shown hereafter that a rock of this nature in the immediate flanks of Caer-Caradoc, passes into what the author terms 'volcanic grit,' proving that the deposition of this part of the series was contemporaneous with the diffusion, if not the eruption, of igneous materials. Another remarkable rock in this series, is a silicious conglomerate, passing into continuous beds of quartz, which is found in so many instances in immediate apposition with the trappean rocks, where the latter cut through the sandstones, as to leave no doubt that its change of character has been the effect of heat. The author's observations prove this change is exhibited under such a variety of forms, and in so many different situations, as to point out the true cause, and to enable him to explain the production of the quartzose masses at Bromsgrove Lickie, and other places, previously obscure. Hoar Edge, a remarkable ridge on the east of Caer-Caradoc, is quartz rock of this description; the beds, altogether about fifty feet in thickness, having been altered by their proximity to the great Plutonic outburst of the Caradoc range itself.

The Caradoc sandstones appear in large mountain masses, in Montgomery and Denbigh shires; where their relations have been investigated by the author with great diligence and success. In Carmarthenshire, a singular tract around Noeth-früg exhibits some very extraordinary contortions of this group;-the place of which, in the system, is clearly determined by the abundance of the characteristic Pentameri, and other fossils; while the proofs of Plutonic action appear in the existence of planes of cleavage, distinct from, yet nearly resembling, those of stratification-and, in some cases, even cutting through the organic remains! These cleavage planes are always parallel, while the surfaces of the beds are often curved. There is no spot, indeed, Mr Murchison remarks, in which the distinctions between cleavage and joints are better defined than in this rugged tract.

It is facts like this which lead us to doubt, whether some of the slaty masses without fossils, now called Cambrian, may not have been originally Silurian rocks, in which the characters have been either wholly or in part defaced by Plutonic action.

7. The Llandeilo Flags-the lowest of the Silurians-are not seen in that part of Shropshire which affords the clearest type of the superior formations. The group, therefore, which forms the base of the system, (if it can at present be said to have a base at all,) has been named from the town of Llandeilo, in Carmarthenshire, where these flags are very extensively developed; consisting of hard dark-grey or indigo-coloured grit, sometimes slightly micaceous, frequently calcareous—with veins of white crystallized carbonate of lime passing occasionally into an impure limestone. They are specially distinguished by containing the large Trilobites, Asaphus Buchii, and a Tyrannus.

These flags occupy ridges which pass under the equivalents of the Caradoc, and in several places graduate downwards into the Cambrian strata.

On the west of Ludlow and Wenlock, the outburst of the trappean hills breaks through and cuts off the lower Caradoc strata; but in several points north of Carmarthenshire, darkcoloured flags, with characteristic Asaphi, rise from beneath the Caradoc sandstones, and repose on Cambrian rocks. Of the order, therefore, in this lowest division of the Silurian system, there can be no doubt; and although the space occupied by the Llandeilos, with reference to that of the Caradoc, is small, the author thinks it better not to merge the former, as a mere subdivision, in the Caradoc formation; since they are marked both by lithological characters and by peculiar fossils, and, both in Carmarthenshire and Pembrokeshire, really occupy a larger proportionate space than the latter formation.

On the south-west of Builth, the Silurian tract is suddenly reduced to a narrow strip, which skirts the north-western border of the old red sandstone thence into

lower Silurians progressively occupying a greater portion of this space in advancing westward; while the upper are comparatively insignificant in extent, and remarkably altered in lithological aspect. Between Llandovery and Llandeilo, the interchange of character in the two divisions is very conspicuous-the shales, which in Salop are more or less incoherent mudstones,' being here represented by compact sandstone; while the hard Caradoc sandstone of Shropshire, has passed in some places into incoherent schist. It is about midway on the course of this reduced band, that the author has taken the type of his Llandeilo group. The tract is on the course of the river Towey, south-west of Llangaddoc; where dark silicious flags, underlying the great mass of Caradoc

sandstone, are well seen at Llandeilo and Dynevor Park. The beds are sufficiently distinguished by their characteristic Asaphi; but throughout their range, between Llandeilo and Carmarthen, a space of about fifteen miles in length, and from half a mile to two miles in width, they have been singularly distributed, often exhibiting divergent strikes and reversed dips. About Llandeilo especially, the dislocations are for the most part such, that the strata are vertical, or very highly inclined.

These lower schists, in their progress westward, contain no calcareous beds; but beyond Saint Clair, on the north of the Taf, a prominent ridge of limestone rises towards the south, containing three remarkable bands, altogether 200 feet thick-the uppermost band alone being seventy feet, and one bed of about ten feet consisting wholly of an aggregate of shells. This limestone, therefore, is thicker than any calcareous mass of this age previously noticed in the Silurian series, and rivals even the Wenlock near Coalbrook Dale. The fossils are, Crinoidea in abundance, several species of Orthis, Corals, and Asaphus Buchii. Mount Pleasant, further north, the black schist contains casts, much contorted, of several shells; among which are a Navicula, (the lowest in situation hitherto known of that genus,) a Spirifer, an Euomphalus, and an Orthis;-and these Mr Murchison regards as occupying the base of the Llandeilo formation here.

Such is the Silurian system,' which it has been the principal object of the work before us to make known. It will be obvious, that, considered even locally-if there had been no corresponding groups in other places-it forms so large and well characterized a portion of this country, as to deserve the attentive study of every British geologist. But it would be contrary to all analogy to suppose that a series of strata so extensive should be confined to any single region; and we shall find that the system is in fact distributed very widely in distant quarters of the globe.

In the chapters which follow the general description of the system, (from chap. 20 to the end of 42,) the author conducts his reader successively through the several portions of his district; giving in each locality a view of the variations of their mineral characters and fossil contents, and of the stratigraphic and volcanic phenomena which they respectively exhibit. Thus he has described the Breidon hills; the trap rocks of Montgomery, Radnor, Brecknock, and partly of Pembrokeshire. Thence returning eastward, he gives an account of obtruded portions of the Silurians forced up through the incumbent groups of strata; the remarkable anticlinal ridges of Castle Madoc and Corny-fan on the north of Brecknock-those of Walsall, Dudley, the Abberly and Malvern ranges, the beautiful circular valley

of elevation at Woolhope, (of which we wish we could have given the detail,) and the elevated range thence to Mayhill, with that on the west of Usk; and finally, of the very complex district of Tortworth near Wotton-under Edge: the correct anatomy of any of which districts it would have been impossible to decipher, without a previous acquaintance with the Upper Silurian series as exhibited near Aymestry.

It is only by the perusal of this portion of the volume that a just conception can be formed of its value. No geological book, that we now know of, gives so much information, with such a readable subordination of detail to general principles. Some repetitions there are, and some occasional redundancies may be cut down-among which the author perhaps will not agree with us in placing certain compliments to contributors and friends, of various degree-our objection to which is quite consistent with a wish for the recognition of preceding labours, by a statement of what each enquirer had done. But the defects, altogether, are slight in proportion to the extent of the work; and the execution, throughout, is very creditable to the author.

We do not condemn the expensive form of the book since it has been favourable to the truth and correctness of the sections, which especially required a large scale for distinctness and effect. But when the present edition has been disposed of, we trust that it will be reprinted in a less expensive form; which will be facilitated by the employment of the plates already engraved. new edition, the author might also describe the best authenticated equivalents of his groups in other countries, and enlarge his introductory chapter by giving the history of his district in detail.

In a

The Second Part of this volume, devoted to the organic remains discovered within the groups described in the first, is principally the work of the several naturalists, who, at the request of the author, undertook the description of the fossils which he from time to time submitted to their inspection. It contains also the maps, and geological sections, and plates of the Silurian fossils; and is certainly one of the most valuable presents which the Palæontologist has ever received. Each of the contributors writes in his own department, in his own language; Mr Murchison having here wisely acted only the part of a general, who chooses the commanders of brigades and detachments, and forms the plan of the campaign; leaving to each chosen officer the conduct and the glory attending upon his own operations. The Fishes are described by M. Agassiz; the Mollusca and Conchifera by Mr Sowerby; the Corals by Mr Lonsdale; the Crustacea by the author himself, with valuable observations by Mr W. S. M'Leay: -several departments also have received illustration from Mr

Broderip, Professor John Phillips, Mr C. Stokes, Dr Beck of Copenhagen, Dr Milne Edwards, Mr König, and Mr M'Leay. Our readers will, after this enumeration, need no assurance that this part of the work is very ably executed.

The general map connected with the volume is excellent, but we regret that the mountains have not been represented in the engraving; and should have wished that two copies of the map should be given, one without colour, exhibiting the natural features of the country, which colours always disguise and obscure. The geological sections are equal to any thing of the kind we have seen; and the general principles of the work are well illustrated by two abstract sections appended to the map; onewhich we have copied at the top of the table subjoined to this article-representing, simply, the stratigraphic succession of the groups; the other illustrating the (supposed) volcanic and Plutonic operations of successive periods, which have disturbed or invaded, and in some cases mixed themselves with the sedimentary deposits during their deposition. The first of these sections is no more than an exposition of facts; the latter is founded upon theory; but so confirmed as to have nearly the force of reality.

The allotment of the description of the fossils, in this second portion of the work, very well exemplifies the advantages of that division of labour which we have mentioned in a former article, as called for by the advancing progress of geology.* It can but rarely happen, in the present state of natural science, that the same person can be possessed of great practical skill in geology, and of sufficient acquaintance with natural history, in its various departments, or even with conchology alone, to be enabled to illustrate competently the collections made by himself. A general acquaintance with each department he must have, and the more of this knowledge he can obtain, the better; but his primary and essential business is more allied to that of the physical geographer and topographic engineer, than of the naturalist: Its object is to ascertain the mineral structure of the earth; and, interesting and delightful as are the enquiries connected with the study of fossils, the geologist employs his natural history, in the first instance, simply as an instrument of stratigraphical arrangement and identification. We should be sorry to be misunderstood upon this point; or to be supposed either to undervalue, or to depreciate the importance of Palæontology to the science of the earth; but it is only a department, which can never supplant or supply the primary and essential necessity for physical, geometrical, and mineralogical research; and it may be useful perhaps to state

*Vol. lxix. p. 431,