great value of the service to science which Sir Charles Lyell performed when he proposed it.

Closely connected with the general subject of existing causes, and forming a very important branch of the inquiry, are the phenomena connected with volcanic eruptions and the formation of cones and craters. Sir Charles Lyell has always taken a prominent part in the discussions which have from time to time arisen respecting certain of these phenomena, and more especially in the controversy between the partisans of the "crater of elevation" and "crater of eruption" theories. The old theory of the formation of volcanic cones was that a vent having been produced by the fracture of the earth's crust (which may have been attended with some degree of upheaval and dislocation), the volcanic materials subsequently ejected gradually formed a conical mound, having a depression in the centre. This mound, or "volcanic cone," is supposed to be composed chiefly of ashes and scoria, which have been ejected into the air, and on falling have naturally arranged themselves in the manner stated. The eruption of dykes and streams of solid lava from the newly formed crater assists in giving solidity to the cone, although it frequently destroys its symmetry by breaking down the walls of one side of the crater. This explanation has been termed the "crater of eruption" theory, and is the one which was most generally received until the celebrated Leopold von Buch propounded the opposing theory of "craters of elevation," an idea which was adopted by Humboldt, and therefore became generally received. Sir Charles Lyell and Mr. Poulett Scrope have always been consistent in their opposition to it; and it is, perhaps, entirely owing to their united exertions that it has now fallen so much into disrepute.

The "crater of elevation" theory may be thus stated:-A vent having been formed in the earth's crust, volcanic materials-lava, ashes, and scoria-are ejected and spread horizontally over the surface, the cone being subsequently formed by sudden inflation and upheaval from beneath.

Sir Charles Lyell devotes several pages in the 'Principles' to the refutation of this theory, and it may be as well to enumerate the chief points of his argument. In the first place, although upheaved strata of various ages occur all over the world, no single instance can be pointed out in which the upheaval has produced a form comparable to that of a truncated volcanic cone. Sir Charles Lyell therefore asks, "Are we then called upon to believe that whenever elastic fluids generated in the subterranean regions burst through horizontal strata, so as to upheave them in the peculiar manner before adverted to, they always select, as if from choice, those spots of comparatively insignificant area where a certain quantity of volcanic matter happens to lie, while they carefully

avoid purely lacustrine and marine strata, although they often lie immediately contiguous?" Secondly, it is in accordance with all analogy to expect that if these great volcanic cones were upheaved after the ejection of the matter composing them, their sides would be fractured and the volcanic strata shattered and disturbed in a considerable degree. But the reverse is the case, for of all isolated hills volcanic cones are the most symmetrical in form, and regular in the arrangement of their constituent materials.

Of late years it has been asserted that volcanoes could not have been formed by "eruption," because solid lava could not consolidate on a slope greater than three degrees, nor vesicular lava on a greater inclination than five degrees. But Sir Charles Lyell proved that this is an error as to a matter of fact. He showed that several of the lavas of Etna of known date have formed continuous beds of compact stone on slopes of 15, 36, and 38 degrees, and in one instance (the lava of 1852) of 40 degrees. Other volcanic cones, such as the island of Palma, yielded similar evidence, so that this objection to the "eruption" theory has been fully answered. The objections to the "elevation" theory have not; they rest on a wider basis, so they probably never will. The form of a volcanic cone is, moreover, precisely that which would be produced by the falling of materials thrown vertically into the air from a central vent.

Sir Charles Lyell has naturally watched with great interest the recent discussions on subaerial phenomena, more especially those on the mode of formation of lake basins and on the origin of valleys and the denudation of the Weald. His latest published examination of these questions is contained in the sixth edition of his 'Elements of Geology,' but they will probably be more fully discussed in the forthcoming tenth edition of the 'Principles.'

In the first edition of the latter work Sir Charles Lyell taught that the Wealden area had been denuded by the sea, to which agent he also ascribed the formation of the chalk escarpments; but he referred the formation of the transverse valleys to the action of rivers running along lines of fracture. Professor Ramsay and others have recently contended that "rain and rivers" and other subaerial agents have produced all the surface-features, not only of the Wealden region, but also of the whole terrestrial surface of the globe, excepting of course volcanic cones and craters. In the opinion of the advocates of this theory the sea has planed off the surface of the land as it emerged, and this form has been termed by Professor Ramsay the "plane of marine denudation." All the existing physical features have been since produced by subaerial erosion. This theory appears to go as much too far in one direction *Principles,' first edit., vol. i., p. 387. + Philosophical Transactions,' 1858.

as its ultra-antagonist, the theory of marine denudation, pur et simple, does in the other; and it is not surprising that Sir Charles Lyell should refuse to give his support to either. It is perfectly possible that atmospheric causes may have produced a greater effect in particular regions than even Sir Charles himself was able to prove in the Principles; but that is quite a different issue, and merely a further proof of the doctrine of uniformity which he has advocated for so many years.

It would occupy too much space to recount the arguments that may be urged in support of the different theories of erosion and denudation; but it may be remarked generally, that phenomena of so varied a character are not, as a rule, referable to the same cause. Certain valleys generally considered to have been scooped out by the sea may have been excavated by rain and rivers, or vice versá; but a multitude of such instances, unless they embrace every possible character of valley and circumstance of occurrence, is not sufficient to warrant the general conclusion that all valleys have been formed by one agent, or by the other.

Another phase of the question is that respecting the meaning to be attached to the expression "form of the ground," this having been very recently the subject of discussion. If the very latest and smallest modifications of the surface are taken into account, of course the present "form of the ground" is due entirely to atmospheric agencies, not excepting volcanic cones and craters; but if this interpretation be insisted on,-why scientific discussion has degenerated into quibbling.

The theory of the formation of lake-basins by glacial erosion is fundamentally new, and has received from Sir Charles Lyell, in the last edition of the Elements,' and in a work to which we have not yet referred,* a fuller examination than the "subaerial denudation" hypothesis, which is merely an old notion revived in an overgrown shape. Sir Charles Lyell is no advocate of the theory that lake-basins have been scooped out by huge glaciers; and in the works we have mentioned he has fully stated the objections which appear to him to render it improbable. He admits, of course, that heavy masses of ice creeping for ages over a surface of dry land must often, by their grinding action, produce depressions in consequence of the different degrees of resistance offered by rocks of unequal hardness;" but the objections to any long continuance of this scooping action on any particular spot are the greater the larger and deeper the lake-basin to be accounted for, because to excavate such a depression a power is required "capable of acting with a considerable degree of uniformity on masses of varying powers of resistance." In opposition to the view that the great Swiss and Italian lake-basins were scooped out by glaciers Sir 'Antiquity of Man,' p. 309.

Charles Lyell has brought forward several arguments, especially, (1) that "several of the great lakes are by no means in the position which they ought to have taken had they been scooped out by the pressure and onward motion of the extinct glaciers;" (2) that lakes of the first magnitude do not occur "in several areas where they ought to exist if the enormous glaciers which once occupied those spaces had possessed the deep excavating power ascribed to them;" (3) that the presence of patches of preglacial freshwater formations in some Alpine valleys, e. g. on the borders of the Lake of Zurich, prove that some of the lakes must have existed before the glacial period.

Sir Charles Lyell seems, however, in this instance, more fortunate in opposition than in proposition. He has shown that the "erosive power of ice was not required to produce lake-basins on a large scale," by means of the preglacial lacustrine formations of the Lake of Zurich. Some other cause must then have produced them if glaciers did not excavate them, and Sir Charles Lyell suggests "unequal movements of upheaval and subsidence." This theory ought to be capable of proof or refutation by geological surveyors, and no doubt it will sooner or later be submitted to the test; but until that is done little more can be said about it, than that it does not enlist in its favour the sympathies of those who have been trained by Sir Charles Lyell himself to the application of the doctrine of Uniformity.

The Antiquity of Man' was published as a résumé of the evidence which has recently been accumulated in favour of the contemporaneity of Man with certain extinct Mammalia. It was avowedly a compilation; but it contains a large mass of matter drawn from a variety of sources, and tending to strengthen the evidence in favour of Man having existed on the earth in Postpliocene times. Perhaps not even the Principles' exhibits more clearly the author's wonderful faculty of "assimilation," as Dr. Fitton called it, of turning anything and everything into good geology. But it is unnecessary for us to discuss this subject at greater length, except incidentally, as being one of the last discoveries bearing on a view of the succession of life in time which Sir Charles Lyell has persistently maintained ever since the commencement of his distinguished career.

Negative evidence has always been a battle-ground for geologists holding opposite views, and it is only of late years that its use has fallen considerably in estimation. The experience of the last halfcentury has taught geologists that it is highly unphilosophical, and positively unsafe, to assume that any class of organisms has not existed at any particular period, or that there is a total break in the succession of life on the earth at any horizon in the geological scale, merely because we have no positive evidence in proof of the

contrary. But Sir Charles Lyell can claim the merit of having foreseen the unstable nature of conclusions based on ignorance, for in the first edition of the Principles' he contended that the apparent breaks in the continuity of geological periods are due to our imperfect information, and do not really exist in nature; and also that the organic remains imbedded in known deposits do not represent the whole of the earth's inhabitants during those periods or in those regions; and he devoted some considerable space to the illustration of these views, in contrast with the then prevalent doctrine of catastrophes.*

In those days Lamarck's hypothesis of progressive development by transmutation of species excited a great deal of discussion; as also did the theory of the successive appearance on the earth's surface of more and more highly organized animals and plants. In support of the latter view, geologists appealed with alacrity to the fossils discovered in different deposits as affording a positive proof of its truth; and they thus endeavoured to define the order of nature, and to assign to each class of organisms the period of its birth. But Sir Charles Lyell contended† that at that time there was "no foundation in geological facts, for the popular theory of the successive development of the animal and vegetable world, from the simplest to the most perfect forms." And although subsequent discoveries have abundantly justified Sir Charles Lyell's protest against invoking negative evidence, to prove that this or that period witnessed the creation of such or such a class of organisms, he has at last admitted that the successive development theory is not much affected by successive discoveries, and is probably necessary in the present state of science. It appears to us, however, not a little mischievous, in so far as it encourages an appeal to negative evidence, as was amusingly illustrated in 1851 by the late Professor Edward Forbes, in reference to the discovery of Pulmonifera in the Purbeck beds, "the (supposed) non-existence of which during the Secondary epoch has called forth not a few prematurely wise comments in geological works."

Agassiz just had given his bail,

"Twas adverse to creation,

That there should live pulmoniferous snail,
Before the chalk formation."§

Since then Pulmonifera have been discovered in Carboniferous deposits, and the history of nearly every group of animals contains a record of similar premature conclusions and their subsequent refutation.

* See also his Presidental Address to the Geological Society in 1851, passim.

+ Principles,' 1st edit., vol i., p. 153.

Antiquity of Man,' p. 405.

§ Wilson and Geikie's Memoir of Edward Forbes,' p. 461.