« EelmineJätka »
thus distinctly what appears to us to be the correct view of this matter, as the fashion of the day seems rather to run in an opposite direction, and to give an undue proportionate value to the study of fossils alone.
It would be unjust to the authors of the excellent descriptions and disquisitions in the Second Part, to give here such a very imperfect abstract of them as our limits would allow. Referring to the Table for a full enumeration of the fossils, we must confine ourselves to some general remarks, which are sufficiently obvious : -1. In descending from the carboniferous strata, after passing over a thickness of no less than from eight or ten thousand feet of old red sandstone, almost barren of organic remains, and at one time considered as altogether destitute of them—we come, at once, upon a series of strata, continued thence to a depth of several hundred feet, which abound throughout in fossils, and include some beds almost entirely composed of them.
2. The great majority of the bodies thus discovered, are of very peculiar character. The Serpulites, the delicately striated Leptæne, the Atrype, Spirifers, Orthises, and numerous Terebratula, distributed in profusion almost throughout the Silurian system, form an assemblage of a very remarkable general aspect. Many of the Orthocerata are also of very unusual appearance; while the genera Phragmoceras and Lituites, have not yet been found in any superior groups. Of the Trilobites—a tribe long since known from the specimens obtained at Dudley, and which here occupies five plates—many species, and even genera, are altogether new; as are likewise many of the corals, in the four plates lithographed by Mr Scharf; while, among the shells, the Pentameri of the Aymestry and the Caradoc limestones, and several others, are known only in the Silurian series.
On this head, we refer our readers to the short introductory chapter of the second part, (xliii. p. 581,) where the author,
restricting himself to that field of enquiry with which he is • conversant,' maintains that the fossils of the Silurian system,
amounting in all to about 350 species, are, with the excep• tion of a very few, essentially distinct from any of the numerous and well-defined fossils of the carboniferous system; and further, that the old red sandstone, which separates these two systems, is • also characterized by fossils peculiar to itself. If the naturalist will compare the figures of the only two works yet published upon
the older fossiliferous rocks—that of Professor Phillips on the carboniferous system, and this work on the Silurian-he • will at once,' says the author, see the truth of my position.'
3. With the new forms of ancient life above mentioned, and others equally strange— Tentaculites, Graptolites, Ischadites, an
VOL. LXXIII. NO. CXLVII.
cient Annelida, some genera are intermixed-and in the oldest groups—which are frequent in the newer secondary rocks; as Trochus, Turbo, Avicula, Pleurotomaria, Pullastra, Nucula, Lingula, Natica, Cardium, Mya, some of the Silurian species of which have a very modern aspect. While of other genera, common also to the ancient and modern groups, Bellerophon, Euomphalus, &c.the Silurian species are of unusual forms. The author, however, conceives that it is proved as a general fact that as we descend • into the older strata, we meet with fresh types of animals; since, • although two or three species of upper Silurian shells may be • detected in the lower Silurian rocks, the mass of organic re
mains in each group is very distinct;—and the distinction is most
strikingly maintained by the crustaceans; no trilobite of one 'group has, as yet, been detected in any other.'-(P. 640.)
4. Mr Murchison remarks, that although the multitude of individuals in the older fossiliferous strata is often very great, the number of species is much smaller than in many of the more recent deposits.t-(P. 583.)
The CAMBRIAN STRATA, beneath the Llandeilos, or lowest of the Silurians, do not come within the immediate object of this volume; but an acquaintance with them is necessary to a just view of some of the relations of the latter groups. It is, in fact, extremely doubtful whether the two systems onght not to be united in one great suite, since they are not separable by any
* It is not impossible that instances of this anticipation, as it were, in the appearance of genera or species at one time supposed to be confined to the more recent strata, may become more frequent, as the fossil contents of our ancient groups are more closely examined. A paper has just been published by Mr Lyell, on the discovery, by MM. Deslongchamps and Tesson,
in the lias or inferior oolite, near Caen, in Normandy, of two species of Conus-a genus abundant in the tertiary strata, but previously known to occur only in two examples so low down as even the upper chalk ; this new situation being deeper, and more ancient in geological chronology, by nearly the entire thickness of the chalk and colitic systems, which cannot be less than 3000 feet. This case, like that of the celebrated Marsupials of Stonesfield, proves the fallibility of negative propositions in matters of natural history.-(Ann. and Mag. of Nat. Hist. Dec. 1840, p. 292, 296.)
† A similar observation has been made with reference to the estuarine and fresh-water deposits, of the comparatively modern Wealden of Kent and Sussex, in which the species are few in number, though vast multitudes of individuals are found. The existing land and fresh-water shells, we believe, in all climates, consist but of few genera and species ; the individuals being more numerous in proportion.
lithological boundary, and Silurian fossils have been already found within the territory which at one time was considered as Cambrian.
The best examples of transition into the Cambrians, occur in Carmarthenshire, near Llangathen and Grongaer-hill; where the Llandeilo flags pass, on the north-west, into concretions of limestone, first alternating with grit, and containing a few corals and casts of Encrinites ; whence there is an imperceptible passage into black schist, void of fossils, which both Professor Sedgwick and the author regard as the connecting link between the Cambrian and the Silurian systems. A similar succession is observable in Denbigh and Pembroke shires.
From the hills near Llanwrtyd, about ten miles west of Builth, and sixteen north-west of Brecknock, the Cambrian rocks range to the south-west, through mountainous and sterile tracts; and afterwards, from about Carmarthen, are deflected westwards, in common with the Silurian formations :- If these rocks have a • tolerably well-defined boundary in that part of their eastern fron• tier (Noeth-früg, Llandeilo, and Grongaer,) where they have
been described as passing into the younger deposits, they have no • lines of demarcation or division within themselves, and extend over two-thirds of Carmarthenshire without any changes in their mineral structure.'_ The separation, it is added, of the lower • Silurian rocks from the upper Cambrian, has been generally effected . [in this work] by assigning to the former those beds which contain fossils, and to the latter those which do not. For, although ani'mal remains occur in the Cambrian strata in many
other parts of England and Wales, nature has here afforded us no such evidences, since the incoherent schists near the base of the Silurian system, and those which extend over so large a portion of the region of slaty Cambrian rocks, are lithologically inseparable. As we ascend in the higher and more arid regions of the north-west, ribs of
grit and sandstone begin to alternate with the slaty schists; and • finally, the beds of schist
, becoming harder, have glossy laminæ, are penetrated by thin veins of white quartz, and put on more the characters of slate.'-(P. 359, 360.)
In the remoter tract last mentioned, occur those remarkable examples, pointed out by Professor Sedgwick,* of slaty cleavage pervading whole ranges of mountains, and cutting indiscriminately through strata variously inclined and inflected.
"I examined,” Mr Murchison says, o these Cambrian rocks towards the interior, by traverses from Llandovery to Llampeter, and from St
* Geological Transactions, (20 Series.) Vol. iii. pp. 469-480.
Clears to Newcastle Emlyn'-[well chosen lines of section, as being at right angles to the general strike ;]— but in no portion of the wide space between these places, have I detected any striking variety of mineral structure; the whole tract being occupied by schists, grits, and sandstones, more or less impressed with a slaty cleavage, ranging generally from north-east to south-west, and dipping to the north-west.'-(P. 361.)
In Pembrokeshire, perhaps the Cambrians are separated from the lower Silurian rocks by a somewhat more definite change of character than any we have yet mentioned. The sequence at the junction being as follows, in a descending order :-(a) the uppermost beds consist—as in the sections already mentioned of black shivery schist, without fossils, and of great thickness. These are succeeded on the north by (b,) hard and thick flagstones—mineralogicallygrauwacke: (c) hard, dark, purple and green close-grained sandstones, perfectly representing the rocks of the Long Mynd in Shropshire, and of the Lammermuir hills in Scotland : (d) the oldest group in Pembrokeshire is schist, rising from beneath the last-mentioned division : it contains roofing slates, with many courses of sandstone, passing into quartz rock, and traversed by numerous veins of quartz. It
may be satisfactory to our readers to enumerate here, the principal components of the Cambrian system in Wales and Cumberland, on the authority of Professor Sedgwick, from a paper read during the meeting of the British Association in 1835; in which the groups of slate rocks, in a descending order, are stated to be as follows :-(1.) Upper Cambrian group. pies the greatest part of the chain of the Berwyns, and is thence expanded through a considerable portion of South Wales, and is connected with the Silurian Llandeilo flags, but contains much less calcareous matter and fewer organic remains. A perfect slaty cleavage is often observed in it, transverse to the stratification; but other parts are of a coarse mechanical texture. (2.) Middle Cambrian. This composes all the higher mountains of Carnarvonshire and Merionethshire, and abounds in roofingslate, interstratified with masses of porphyry. It contains (as at the top of Snowdon) a few organic remains, and some highly calcareous slates, but no continuous beds of limestone. The same group, but without organic remains, is greatly developed in Cumberland. (3.) Lower Cambrian, occupying the southwest coast of Carnarvonshire, and a great part of Anglesea. It consists chiefly of chlorite-schist, passing into mica-schist and slaty quartz rock, with subordinate masses of serpentine and
white granular limestone-no organic remains. Beneath the middle Cambrian there occurs in Cumberland a great formation of clayslate, without calcareous matter, and without organic remains. It passes downwards into chiastolite-slate, mica-slate, &c., and gneiss, which rest immediately on granite. Whether it is to be placed on the exact parallel of the lower Cambrian of Wales, is still uncertain.
Our information respecting the organic remains of the Cambrian system is very limited.
limited. In England, hitherto, they have been rare. The relations of the beds containing moulds of shells on Snowdon, and of the Bala limestone, are still doubtful; and the supposed Cambrian fossils from some other places, we have reason to think, will prove to be Silurian. Almost the only well authenticated remains from the latter strata, are the traces of fossil Annelida, or sea-worms, from Lampeter, in North Wales, described at the close of the volume before us, by Mr W. M'Leay.
From the series of facts above briefly stated, it appears that, although the chlorite and mica-slates, &c., at the bottom of the Cambrian system, afford a strong contrast to the soft shales and mudstones of the upper Silurian groups, there is a gradual transition of character by which these extremes are insensibly connected : And as the lowest crystalline slates are, by common consent, regarded as metamorphic-while the change, whatever it has been, which has produced the phenomena of cleavage, pervades the whole of the Cambrians, and is perceptible even in the lower fossiliferous Silurian beds—it is very difficult to decide how far the original sedimentary character of any portion of the strata has remained unaltered. A geologist who reasons on merely lithologic grounds, might claim the whole of the two systems for the Cambrian or Silurian, according to the point from whence his observations began; and until fossils, distinctly and widely different from the Silurian, be found among the lower and middle Cambrians, the division of the systems seems to be merely conventional, and matter of temporary convenience.
From the almost universal prevalence of cleavage, and other metamorphic changes, as well as of great mechanical disturbance in the ancient groups of England, it is not probable that the grounds for a determination of the natural divisions of these great stratified masses will be obtained in this country. They should be expected, perhaps, and sought for in regions--if there be any such—where the groups have retained, along with their original sedimentary condition, the position also in which they were deposited-at least as little altered by elevation or disturbance as is consistent with the fact of their being visible to our