pend upon the greater or less thickness of the strata and their diversity of lithological structure?

If, then, greater thickness of the masses be abandoned as a reason for a separate name, some geologists night contend that a physical separation of the upper and lower groups, or the unconformability of the one to the other, would afford grounds for such a distinction. But even this feature is wanting, in reference to the two groups characterised respectively by Upper and Lower Silurian fossils. There are, indeed, districts in which one portion of the Upper Silurian group is unconformable to another; and, again, lines of dislocation producing unconformity occasionally affect the subordinate members of the lowest group itself, which Professor Sedgwick recognises as one natural whole. But we already know from the survey of the Government geologists, that all the Silurian strata roll over in conformable folds throughout South Wales, and that there is no general break between the masses occupied by Upper and Lower Silurian fossils in Wales, any more than within the limits of the Silurian region first chosen as a pattern. Indeed, I have strong grounds for believing, that the very rocks of Bala, about which so much discussion has taken place, will prove to be the physical equivalents of the schists, flags, limestones, and sandstones which, in South Wales, have been described by me as Llandeilo flags.

*

There being, then, no unconformity between my Silurian and the Cambrian of Professor Sedgwick, the only remaining ground for changing the name, is the opinion which he seems to entertain, that the Silurian system, as originally described, is in reality made up of two natural history groups, and ought therefore to have two names. Or this point also, it is scarcely necessary that I should go beyond the clear evidences recently afforded by the Professor himself, of the great interchange of fossils between the Upper and Lower Silurian groups, to convince every one that they are so knit together in Britain, as to be geologically inseparable.† When I published the Silurian System,' I then knew that a limited number of species only passed from the upper to the lower group, but in succeeding years I learnt, that many more were common to the two groups, both in Great Britain and in other countries. My last memoir on Gothland has shewn, that out of 74 species of shells and crustacea in the Upper Silurian rocks of that island (46 of which are British Upper Silurian forms), 9 at least range into the Lower Silurian rocks of Britain, whilst 14 is the number, if the Lower Silurian type of Northern

*See also Quarterly Journal Geol. Soc., ante, vol. ii.

†The researches of the Government Geological Surveyors will presently bring to light facts which will place beyond all doubt, that even in North Wales or Cambria itself, many Upper Silurian forms are intermixed with those of Lower Silurian age; in short, Professor Edward Forbes has assured me, that in North Wales there is but one natural history system only.

Europe be included. On the plain fact, therefore; that there are many species of Trilobites, Orthida, and other shells which unite the two groups, I maintain that the Lower Silurian cannot be viewed as a system independent of the Upper.

But it is not on the duration or passage of species from the one group to the other, that I alone depend for the conservation of the zoological unity of my system. The qualifications and character of what I term a system are chiefly based on the assemblage of its classes of animals. Thus, the Silurian was typified as the great system of Trilobites, which crustaceans rapidly dwindle away in the overlying Devonian, and expire in the Carboniferous system. Again, the Silurian system was represented as being the chief centre of Orthidæ, its lower half being specially marked by small species of that genus with simple plaits. It was further spoken of as charged with Graptolites, and also as being the horizon in which certain very peculiar chambered shells are most rife. Of late years its lower strata have been shewn to abound in Cystidea, those simple forms which, chiefly by the labours of Von Buch, have been shewn to be the earliest created forms of the great family of Crinoids. And here I would beg British geologists to attend to the importance of foreign comparisons, if they wish to see rock systems founded on laws of general distribution of animals. Abounding in the Lower Silurian rocks of Scandinavia and Russia, these Cystidea had not been found by myself in the Lower Silurian rocks of Britain, but the researches of the Government Geological Surveyors detected the common species of Northern Europe (Echino-sphærites aurantium) in strata actually described and coloured in the map of the Silurian region by myself as Llandeilo flags, whilst the same observers are now detecting the same in greater quantity in the rocks of Bala and in Ireland. Even whilst I write, I learn that the only strong distinction which was thought to exist between the Upper and Lower Silurian rocks has vanished by the discovery of the defences of cartilaginous fishes of the genus Onchus in the latter, as just announced by Professor Sedgwick; and thus, whilst my view of a period void of vertebrata, founded though it was on very general observation, must be abandoned,* I naturally rejoice in this unexpected additional evidence, whereby the Upper and Lower Silurian rocks are still more firmly united in one system.†

* See Russia in Europe, and the Ural Mountains.

I also learn from Professor E. Forbes, and the geologists of the Government Survey, that they have detected the defence of an Onchus in the limestone near Bala. Professor Sedgwick states, that the species he mentions were found in the Upper Llandeilo flags. Professor Phillips has detected fish-remains in the Wenlock shale, and they had been previously observed by the Rev. C. Brodie in the Wenlock limestone. Although the species of North Wales are not yet described, it is rather remarkable that the Onchus of these Lower Silurian rocks is said to resemble, to a great extent, the O. Murchisoni, Agassiz, of my Ludlow rocks,

Whilst we are considering what are the natural distinctions of a system, or "terrain," in the sense in which I used the word, I beg to be permitted, without further reference to Russia and Scandinavia, to shew how, in another portion of the continent, the Silurian system has recently been applied by a French geologist, M. Barrande, to Bohemia, the country of his adopted residence. The slaty fossiliferous rocks of Germany, like those of Devon and Cornwall are, it is well known, for the most part of Devonian age; but in Bohemia, a long and wide tract consists of a basin of Silurian rocks, the lowest strata of which repose on sedimentary rocks void of fossils, and these again on crystalline schists. With no other guide than my original work, the "Silurian System," M. Barrande, after collecting 600 species of Bohemia fossils, 129 of which species are Trilobites, has, of his own accord, come to the conclusion, that the whole clearly indicate a true Silurian series. The lower half of this series is composed of two stages of quartzose and argillaceous strata, which are not merely referred to the Lower Silurian as a whole, but, through their mineral characters and their Trilobites, and other organic forms, are even severally compared with the Llandeilo flags and Caradoc sandstone. The upper group, eminently calcareous, presents itself in three stages, the lowest of which M. Barrande (after most assiduous examination of the fossils) compares with the Wenlock limestone and shale, the middle with the Lower Ludlow rock (the chambered shells of which strikingly resemble those of the same formation in England), and the third or upper division with the Aymestry limestone and Upper Ludlow rock. I advert, therefore, with pleasure to such labours, because they prove that the detailed descriptions of certain typical Silurian tracts in England have not been unfruitful, even in reference to other and distant European tracts. I further trust that geologists may regard my late memoir on Gothland as a corroboration quite as striking, of the value of the detailed original description of the types of my own country.* * On the other hand, I well know that there are Silurian tracts in the British Isles, which, under different conditions of mineral origin present much fewer points of resemblance to the types of the Silurian region than the above mentioned distant foreign regions.†

But rock systems must not be so formed as to suit one country

*See Quarterly Journal Geol. Soc., vol. iii., p. 1.

Whilst these pages are passing through the press, I have learnt from Mr M'Coy, that several of the Upper and Lower Silurian species of Ireland, which he has described and named. are identical with specimens from Bohemia now in the collection of the University of Cambridge; and together, with this fact, it is curious to remark, that the mineral characters of the rock masses in the two countries coincide; and that in Ireland, as in Bohemia, the Upper Silurian is eminently calcareous, the Lower Silurian sandy, quartzose, and slaty. (See the Silurian fossils of Ireland, 4to, as published by Mr Griffith and Mr M'Coy). -9th April 1847.

only. For if it be granted, that in Britain the Lower can generally be so separated from the Upper Silurian, as to be capable of demarcation on a map, how could the geologist succeed in separating the Upper and Lower Silurian of Christiania in Norway, where these groups, each equally well characterised by fossils as in our own country, fold over together in such small united masses ?*

Having stated that the Upper and Lower Silurian rocks of Europe, including all the fossiliferous strata of Wales, are so knit together by fossils and the transitions of the masses, that they must be viewed as one natural history series, I will now conclude by simply indicating the small area in various countries to which the Silurian system would be reduced, if the meaning of that term were to be changed and restricted to the upper half of the original system.

In England, though prominent in the typical districts of Shropshire and Herefordshire, the Silurian rocks, so dismembered, would occupy a mere band (scarcely to be defined on a general map) in Brecknockshire, Carmarthenshire, and Pembrokeshire; whilst all the broad and specially typical Lower Silurian tracts laid down by me in Salop, Montgomery, Radnor, and South Wales, must be erased, since it is impossible to distinguish them by their organic remains from the groups of Snowdown. In Ireland, from what we already know, whether through the works of Mr Griffith, Captain Portlock, or the labourers of the Government Surveyors, the system would pretty nearly disappear, as the great mass of the Irish lower paleozoic fossils are found to be Lower Silurian.

In Russia in Europe, and in nearly all Scandinavia, Lower Silurian rocks and fossils only prevailing, the very name Silurian would be swept from the map, and the system so attenuated, would there be confined to Gothland and some small Baltic isles!

Now, it must be borne in mind, that even when the lower and upper groups are united in one system as at present, the Silurian rocks of Russia do not occupy one-fifth part of the area of either the Devonian or Carboniferous systems of those regions,† whilst in Germany, the whole system, as at present united, bears an infinitely small proportion to the overlying Devonian group. A glance at the Geological Map of America, in Mr Lyell's work, shews to what a small area, in relation to the other paleozoic rock system, the Silurian would be reduced, if its lower half were abstracted.

Independent, therefore, of the impropriety of mutilating a system established on the community of its zoological contents, the results of such an arrangement would be a violation of the meaning which ought in fairness to be attached to a great natural history period, which was typified as such before the Devonian System was thought of.

There

*See woodcut, Russia in Europe, &c., vol. i., p. 17, and Quarterly Journal

Geol. Society, vol. ii., part 2 (Miscell.), p. 71.

† See General Map, Russia in Europe, and the Ural Mountains.

VOL. XLIII. NO. LXXXV.—JULY 1847.

L

are indeed authors who think, that the Upper Silurian is so linked on to the Devonian, that the former, or a large portion of it, might advantageously be merged in the latter; and if their views prevailed, the only portion of my system or terrain which Professor Sedgwick's proposition leaves to me would also be swallowed up, and thus, by invasions on both sides, the poor Silurian system would be obliterated.

On the principle, however, of strata identified by their fossils, geologists, I hope, agree with me to the conviction in which I abide, that in whatever rocks, and to whatever depths the Lower Silurian types extend, the tracts so characterised must be considered to belong to the "Silurian System."

In contending for the preservation of that system in its unity, I in no way detract from the very great merit of the researches of my friend, Professor Sedgwick, in developing the physical structure, dislocations, slaty impress, fossil characters, and other phenomena of the rocks of North Wales, Cumberland, and Westmoreland.

As already stated, I formerly hoped he would also point out in their lowest division the existence of a distinct zoological type which would have entitled those rocks to a separate name; but having failed to do so, it seems to me manifest, that his "Cambrian" cannot now be sustained by dismembering a fossiliferous system which has been so long established, so largely developed, and so widely applied over the world as the Silurian. Whether geologists will use the word "Cambrian," in reference to still older and often unconformable greywacke lying beneath all the beds with Silurian fossils, it is not for me to determine. My chief object in this communication is to explain how, by the progress of research, the protozoic types of various parts of Northern Europe, including North Wales, have been shewn to be true equivalents of the lower part of a natural system which I proposed twelve years ago, and which geologists of various countries adopted after a careful scrutiny of the evidences on which it was established.

On the Protein Question. By Professor MULDER of Utrecht. Communicated by Professor J. F. W. JOHNSTON.

DURHAM, 17th June 1847.

MY DEAR SIR,-Will you favour me by inserting in your ensuing number, the following additional communication from Professor Mulder on the Protein question? The perusal of it will shew you that we are slowly progressing towards a more perfect knowledge of this important substance.

The state of our knowledge in regard to it is at present nearly as follows :

1. There exists in the parts of animals and plants a substance to