PREFACE.

WHILE resident in Italy during the winters of 181718-19, I had observed with great interest the volcanic phenomena of Vesuvius, Etna, and the Lipari Isles, and paid considerable attention to the structure of the district west of the Apennines, between Santa Fiora in Tuscany and the Bay of Naples, which presents unmistakeable traces of volcanic action on an extensive scale, though no eruption has taken place there within the historical period.

After my return to England, being for some time at Cambridge, I had the advantage of frequent intercourse with the late Professor E. D. Clarke, who was himself well acquainted with volcanic Italy, and Professor Sedgwick, at that time commencing his distinguished career as a geologist. The doctrines of Werner were then so completely in the ascendant that it was considered little better than heresy to dispute any of them. Yet it appeared to me, from the knowledge of igneous rocks I had acquired in Italy, that the dogmatic canon of that school which denied a volcanic origin to the Floetz Trap-rocks (as basalt, clinkstone, and

trachyte were then called), and declared them to be precipitations from some archaic ocean, was signally

erroneous.

My two friends agreed with me in the opinion that the error of the Wernerians in undervaluing, or rather despising altogether as of no appreciable value, the influence of volcanic forces in the production of the rocks that compose the surface of the globe, formed a fatal bar to the progress of sound geological science, which it was above all things desirable to remove.

Being shortly after free to choose my path of travel, I determined to examine with care such evidence upon this point as might probably be found in Auvergne and the neighbouring districts-a country where the products of extinct volcanos are brought into contact with some of the earliest crystalline rocks, as well as with the most recent (tertiary and freshwater) strata.

For this purpose, in the beginning of June, 1821, I established myself at Clermont, the capital of the department of the Puy de Dôme, and passed some months in continual examination of the geology of the neighbourhood; removing from thence, as it became convenient, to the Baths of Mont Dore, Le Puy (Haute Loire), and Aubenas (Ardèche). I afterwards revisited Italy, where I had the good fortune to witness by far the most important eruption of Vesuvius that has occurred within this century-that of October, 1822.

On my return to England in 1823, I published a volume on the Phenomena of Volcanos.'* In that work unfortunately were included some speculations on theoretic cosmogony, which the public mind was not at the time prepared to entertain. Nor'was this, my first attempt at authorship, sufficiently well composed, arranged, or even printed, to secure a fair appreciation for the really sound and, I believe, original views on many points of geological interest, which it contained. I ought, no doubt, to have begun with a description of the striking facts which I was prepared to produce from the volcanic regions of Central France and Italy, in order to pave the way for a favourable reception, or even for a fair hearing, of the theoretic views I had been led from those observations to form.

Indeed this obvious error was pointed out in a very friendly manner by the Quarterly Reviewer of this Memoir on the Geology of Central France,' which was shortly after published. That article was, I believe, the first essay of my distinguished friend Sir Charles Lyell, in the path of geological generalisation which he has since so successfully pursued. And I have sometimes ventured to think that during its composition he may have imbibed that philosophical

Considerations on Volcanos, &c.,' 1825.

+ Quarterly Review for May, 1827.

conviction as to the true method of inquiry into the past history of the globe's surface, namely, through a careful study of the processes actually in operation. upon it, which is the leading principle of his deservedly popular works.*

*This was the pervading idea of both my early works, as will appear from the following passage in the Preface to the 'Considerations on Volcanos,' published in 1825.

"Geology has for its business a knowledge of the physical processes which are in continual or occasional operation within the limits of our planet, and the application of these laws to explain the appearances discovered in our geognostical researches, so as from these materials to deduce conclusions upon its past history.

"The surface of the globe exposes to the eye of the geognost abundant evidence of a variety of changes which appear to have succeeded one another during an incalculable lapse of time. These changes are chiefly,

"1. Variations of relative level between different constituent parts of the surface of the terraqueous globe.

"2. The destruction of former rocks and their reproduction under new forms. "3. The production of new rocks upon the earth's surface.

Geologists have hitherto usually had recourse, for the explanation of these changes, to the supposition of sundry violent and extraordinary catastrophes, cataclysms, or general revolutions.

"As the idea imparted by the terms cataclysm, catastrophe, or revolution, is extremely vague, and may comprehend anything you choose to imagine, it answers for the time as an explanation; that is, it stops further inquiry. But it has the disadvantage of stopping also the advance of the science by involving it in obscurity and confusion.

"If, however, instead of forming guesses as to what may have been the possible causes and nature of these changes, we pursue that which I conceive to be the only legitimate path of geological inquiry, and begin by examining the laws of nature which are actually in force, we cannot but perceive that numerous physical phenomena are going on at this moment on the surface of the globe, by which various changes are produced in its constitution and external character. These processes are principally: I. The atmospheric phenomena, including the laws of the circulation and residence of WATER on the exterior of the globe. II. The action of earthquakes and volcanos. And the changes effected before our eyes, by the operation of these causes on the constitution of the earth's crust, are chiefly,

"1. Changes

My purpose at all events was fulfilled. The Wernerian notion of the aqueous precipitation of " Trap" has since that date never held up its head. And I had good grounds for believing that the publication of the first edition of this Memoir, illustrated by an atlas of maps and drawings, which presented "oculis fidelibus " convincing evidence of the identic origin of ancient sheets of basalt crowning high mountain platforms, and contiguous lava-streams so recent as

"1. Changes of level.

"2. The destruction of some rocks and the reproduction of others from their materials.

"3. The production of rocks de novo from the interior of the globe upon its surface.

"Changes these which, in their general character at least, bear so strong an analogy to those which appear to have occurred in the earlier ages of the world's history, that until the processes which give rise to them have been maturely studied under every shape, and then applied with strict impartiality to explain the appearances in question; and until, after a close investigation, and the most liberal allowance for all possible variations and an unlimited series of ages, they have been found wholly inadequate to the purpose, it would be unphilosophical to have recourse to any gratuitous and unexampled hypotheses for the solution of these analogous facts.

"The study of the processes by which these effects are at present produced on the surface of the globe, forms, therefore, a most important, but unfortunately most neglected, branch of geology."

I went on to say, that the work then produced was intended as a contribution to our knowledge of that division of the subject which relates to "the phenomena produced on the exterior of the globe by the development of its internal and subterranean activity," leaving to others, or to a future work, a corresponding inquiry into "the laws which determine the atmospheric influences, or the decomposition of rocks by air, light, electricity, or magnetism, and the conduct and mechanical effects of water on the surface of the globe, and on its solid parts."

Let me hint in passing, that this latter branch of inquiry has not yet, perhaps, been sufficiently pursued in detail, even by Sir Charles Lyell, or any other geologist.