For instance, the Ptolemaic and Copernican systems of Astronomy were each supplanted in its turn; but does any astronomer pretend that their authors are therefore less worthy of a place in the very front rank of the great discoverers in his science? Therefore, although we admit that the percentage test is logically not faultless, we consider that Sir Charles Lyell is not the less entitled to great credit, and exalted rank as a geologist, for its promulgation more than thirty years ago.

As we have already stated, Sir Charles Lyell gave the names Eocene, Miocene, and Pliocene to his three great divisions of the Tertiary series; and he again subdivided the Pliocene into Older and Newer. As typical formations, he referred to the Eocene division the strata of the London and Paris Basins; to the Miocene, the Faluns of the Loire and the beds of the Superga, near Turin; to the Older Pliocene, the Crag of England and the Subapennine strata of Italy; and to the Newer Pliocene, the Sicilian beds and more recent deposits. He anticipated the future discovery of beds which would lessen the gaps that then existed between the members of these various subdivisions; and, it is almost needless to say, his anticipation has been amply realized. The question for us now to consider is how far these divisions are natural, and how far they are arbitrary. Many geologists would affirm that all divisions of strata are arbitrary, while others would contend for their being mostly natural. If the whole surface of the earth be considered, and if we are supposed to possess a complete knowledge of its geological history, then no doubt all divisions are arbitraryfor there must have been a continuous sequence of deposits. But in the present state of our knowledge-some deposits being unknown (either not explored or submerged) and others destroyed,—it is no doubt true that, for particular areas, while some divisions of strata are quite natural, others are more or less artificial. Now, into which category do these divisions of Eocene, Miocene, and Pliocene fall? Sir Charles Lyell himself would say that they are artificial, as all divisions necessarily must be. But for ourselves we should say that while these divisions are artificial, others may be, as far as Europe or any other region separately is concerned, as purely natural as any in the Geological scale.

A study of the literature of the Tertiary system will reveal the fact that in North Germany and in Austria, where certain portions of the series are extensively developed, geologists have been obliged to invent new terms to designate groups of beds which they have been unable to refer with confidence to any one of Sir Charles Lyell's divisions.

In North Germany, Professor Beyrich has grouped together, under his new term Oligocene, a long succession of beds older than the typical Miocene Faluns, and newer than the Nummulitic

(Middle Eocene) strata of the Paris Basin and other districts. Sir Charles Lyell, however, refers to his Lower Miocene all the beds as far down as the Hempstead series, including that deposit. The remaining strata in question he calls Upper Eocene. Sir Charles has confessed repeatedly that his line is purely arbitrary; but he contends that the other is equally so, and that there is consequently no need of a new term. Few Tertiary paleontologists will, we imagine, agree with him in this, and it certainly seems preferable to curtail the Eocene and Miocene, and interpolate a new group, than to be confessedly reduced to the necessity of drawing a line where there is no physical or paleontological break.

In the Vienna Basin, there exists a very complete series of Miocene (Upper Miocene of Lyell) deposits, passing gradually upwards into newer strata. From the difficulty the Austrian geologists have experienced in defining the upper limit of the Miocene deposits, they have at last been led to abandon the terms Miocene and Pliocene, and to group the whole of the strata embraced within their definitions under the single term Neogene. In endeavours to assign to their place in the series the Tertiary deposits of other regions, questions have been raised as to the value of the distinction between Miocene and Pliocene strata, and some palæontologists have gone so far as to assert that the significance of the terms is far more climatal than chronological; in fact, that in tropical regions it is impossible to say that certain deposits are Miocene and not Pliocene, or vice versa. Under these circumstances it certainly does seem advisable to unite the two divisions, especially for the purpose of assigning to their proper horizon the fossils of low latitudes. The revised classification would then exhibit to the old one of Sir Charles Lyell the relation shown in the following table:

Even this revised classification cannot claim the merit of being entirely natural; but it is certainly nearer that Ultima Thule of systematists than the original one of Sir Charles Lyell. It would indeed be strange if geology had made no progress in this direction for more than thirty years; and the only marvel is that, in a science which makes such gigantic strides, the original classification has not by this time been entirely swept away. The fact that it has not, is, however, conclusive testimony of the reality and

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

is

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," supposed to be composed chiefly of ashes and scoriæ, 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 scoriæ-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

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.