..In like manner, I hope it will not argue any latent bias in favour of the exploded doctrines of the Wernerian system,

In order therefore to form a fair and candid estimate of his scientific merits, we ought to view him at the commencement of his career, or at least carry our ideas as far back as the period at which his school was resorted to by individuals of almost every nation, as the only then existing source of sound and practical information on the subjects which he taught.

Geology indeed, as it was studied at Freyburg, bore at that time about the same relation to its condition elsewhere, as History does to Mythology, or Chemistry to Alchemy; and, if it be objected, that even Werner did not altogether emancipate himself from the fables and chimeras that occupied his brethren elsewhere, it may be answered, that his defenders, at least at the present day, neither claim for him infallibility, nor an exemption from human infirmities.

I believe it may be laid down as a general law, though it is one no doubt sufficiently mortifying, not only that the groundwork of every thing great and good achieved by the human mind in whatever line is laid during the first thirty years of life, but that the opinions and researches, that appear to originate afterwards, have received, for the most part, all but their final development during the same period.

To say therefore, that Werner's geological system partook at first of the imperfections belonging to a new branch of knowledge, and that in his advanced years he felt reluctant to modify it, as a younger man might have done, in proportion to the new light that the science had received, is a reproach indeed, but one which applies too generally, to bear very heavily on his individual reputation.

It is also said, that he gave an undue prominence to theoretical opinions, and inculcated as dogmas, what, after all, ought only to have been brought forward as hypotheses.

But some excuse ought in candour to be made to a popular Lecturer, who, in the warmth of extemporaneous speaking, may sometimes indulge in speculations, which he himself would hardly deem admissible in his published writings; and if his disciples in some cases have chosen to build their faith on figments, which were introduced perhaps chiefly to enliven the dryness of practical details, they, and not their master, are to blame,

Without therefore professing that blind admiration for Werner, which his pupils at one time appear to have entertained, I cannot help considering, that the branch of natural history which he cultivated is greatly indebted to his exertions; and though the time, it must be confessed, is gone by, in which an addiction to the tenets of this or any other School of Geology can be defended, yet I am upon the whole inclined to think, that, up to a certain point in the progress of this science, even the exaggerated opinion entertained with respect to the merits of the Wernerian system may have had its use, as tending to inculcate more fully those principles of classification, and that method of discriminating rocks and minerals, which, with all their imperfections, must be allowed to possess no slight superiority over preceding ones, and to have facilitated upon the whole the advances that have been since made in this branch of knowledge.

if I suspend for the present my opinion with respect to the origin of granite; provided only, that I do not at the same time follow the example of Werner, in interesting myself so far on the opposite side of the question, as to receive with reluctance or incredulity the facts, which may perhaps hereafter serve to place the Plutonic theory on a less questionable foundation.

Indeed if we would avoid in future those oscillations of opinion on matters of geological theory, which, whenever they occur, serve, like vacillation in practical matters, to betray the infant state of the study itself, we must not adopt the opinions of the Plutonists, merely because they may be more probable than those opposed to them, but should be › content to wait, until the evidence in support of them arrives at such a degree of force, as to place the theory on a level in point of credibility with those systems, which are received in other departments of science as established.

One of the questions which require to be more fully elucidated, before the origin of granite can be viewed as determined, is, whether this rock be ever found intruding itself into the midst of modern strata, as might be expected to be the case, if it were analogous in its origin to basalt. One instance has indeed been lately brought forwards, in which this rock is said to rest on a very recent limestone at Predazzo, in the Tyrol. The superposition is affirmed by Maraschini, Boué, and others; but it is questioned by Von Buch, who imagines, 1 believe, a fault to have taken place, which, by turning the strata completely over, has produced the delusive appearance described.

The same uncertainty seems to exist with regard to the formation of serpentine, as to that of granite. In the Lizard district of Cornwall, where rocks of the former kind extensively occur, the impression left upon my mind was, that the origin of the serpentine, greenstone, and clay-slate, was probably the same, and those who will peruse Professor

Sedgwick's paper on that country, will, I think, arrive at the same conclusion.*

Nevertheless in other situations serpentine seems to form dykes, possessing all the characters of igneous injection which distinguish those of trap. My friend, Mr. Lyell, late Secretary to the Geological Society, has described one of this kind in Forfarshire, and Dr. Boué has communicate to me other instances observed by Von Buch and himself, in various parts of the continent.

The existence however of a central heat, which some regard as demonstrated, may appear to lend considerable weight to the Plutonic theory, and accordingly deserves a short notice in this place.

In a late number of the Annals of Philosophy, is an ingenious paper by Sir Alexander Crichton,+ in which the necessity of supposing the earth to be hotter in the interior, than it is at the surface, is inferred from the high temperature which he supposes to have prevailed in the Antediluvian World. This notion, which he entertains in common with Humboldt and other distinguished Naturalists, is chiefly derived from the existence of impressions of tropical plants in the coal strata even of the most northern regions, from which it seems fair to infer, that the climate enjoyed at those periods was equal to that in which these vegetables thrive at present.

Even at a much later epoch than this, during the formation of the beds above the chalk, a tropical climate seems to have prevailed in the latitude of London, as appears from the specimens of cocoa-nut and other analogous vegetable remains found in the Isle of Sheppey; so that, as Mr. Conybeare observes, we may figure to ourselves the high mountain tracts, which at that time had raised their head

* See his paper "On the Physical Structure of the Lizard District," in the Cambridge Phil. Trans.

+ Annals of Philosophy for November and December, 1825.

Conybeare and Phillips' Geology of England and Wales, p. 30.

above the waters, as forming a groupe of spice islands, fre-quented by the turtle and the crocodile.

I feel therefore much less disposed to object to this part of Sir A. Crichton's theory, than to that in which he refers this high temperature to the fusion of the primitive rocks, which he explains in the following manner.

The nucleus of the globe consists, he says, of the metallic bases of the earths and alkalies, which, in the beginning of things, took fire from the contact of air and water, and produced by their combustion granite. The latter, retaining its temperature for a very long period, would impart to the earth a source of heat independent of the solar rays, which must have gone on progressively diminishing down to the present time. But, as the earth must be supposed to have parted with enough of its caloric, to allow of the existence of certain animals as early as the epoch of the transition rocks, how comes it, that it did not sink below the standard of tropical heat by the time the coal formation was created, or, granting it an equinoxial temperature then, ought it not at least to have become too cold for the existence of crocodiles, and the growth of spices in this latitude, at a period so distant as that of the tertiary formations?

Nor can it be contended, that the earth was receiving from time to time fresh accessions of heat by the continuance of the same process which first gave rise to it, for we have no proof of any general eruption of granitic matter having taken place at a period subsequent to that of the transition strata, and, even if we admit, that there are cases, like the still disputed one of Predazzo, in which granite has been thrown up at a comparatively recent period, such local occurrences would have but little influence in modifying the general temperature of the earth's surface.

Neither would it seem altogether satisfactory, if we were to suppose that the heat in the interior of the globe has been kept up by the volcanic action continued from the earliest period down to the present; at least until it shall have been proved, that the temperature is highest in those spots around

which these processes appear to go on with the greatest intensity, as in the neighbourhood of the sea.

The advocates however of a central heat call in to their assistance some observations recorded on the temperature of mines, which seem to shew that the substance of the earth is hotter in proportion to its distance from the surface.

It is one thing to admit the existence of a central heat, and another to decide upon the cause from which it may have arisen; nor am I prepared to deny the truth of the observations appealed to on this subject, which often appear to have been made by unprejudiced persons, and in particular those with respect to the progressive increase in that of the Cornish mines, in proportion to their depth.

Baron Fourrier has lately published a very elaborate treatise on this subject, in which however, if I am rightly informed, he has neglected to consider particularly the local causes of heat in mines, to which their temperature may perhaps be attributed.*

That these often interfere with the results, I feel disposed to believe from my own experience, such as it has been, on this subject; which leads me to the conclusion, that the temperature is often influenced by causes, less obvious, and less easily guarded against, than those arising out of the state of ventilation, the number of workmen employed, &c. &c.

In the course of my travels on the Continent I had repeated opportunities of examining into this subject, and in all the cases, where any remarkable degree of heat was discoverable, detected the presence of a large quantity of pyrites in a state of decomposition.

Such was the case in the mines of Hungary, where I was assured that in one instance, and that not the deepest part of the mine in which it occurred, the workmen were compelled from the heat to wear masks and gloves.

I was struck with observing the same thing in a differen

* I have only seen the Extract from his Memoir in the Annales de Chemie Tom, xxvii. Oct. 1824.