British Quarterly Review, American Edition, Vol. LIV / July and October, 1871

by Various

'It must not be forgotten,' (these are his words) 'that the geological speculations still in vogue respecting the original fluidity of the planet, and the gradual consolidation of its external shell, belong to a period when theoretical ideas were entertained as to the relative age of the crystalline foundations of that shell wholly at variance with the present state of our knowledge. It was formerly imagined that all granite was of very high antiquity, and that rocks, such as gneiss, mica-schist, and clay-slate, were also anterior in date to the existence of organic beings on a habitable surface. It was, moreover, supposed that these primitive formations, as they are called, implied a continual thickening of the crust at the expense of the original fluid nucleus. These notions have been universally abandoned. It is now ascertained that the granites of different regions are by no means all of the same antiquity, and it is hardly possible to prove any one of them to be as old as the oldest known fossil organic remains. It is likewise now admitted, that gneiss and other crystalline strata are sedimentary deposits which have undergone metamorphic action, and they can almost all be demonstrated to be newer than the lately-discovered fossil called Eozoon Canadense.'

"With all deference to one whom we acknowledge to be among the very ablest living geologists, we must say that this language strikes us as more emphatic than the state of the discussion warrants. We do not undertake absolutely to maintain the theory of central heat as explaining the formation of the granitic and metamorphic rocks, but we cannot admit, what Sir Charles seems to imply, that the time has arrived when investigation and experiment on the subject may be relinquished, and the tone of dogmatic confidence assumed. The reasonableness of permitting a certain degree of suspense of judgment regarding it becomes the more evident when we observe that Sir Charles is not prepared to maintain against astronomers that the planet was not originally fluid. 'The astronomer,' he says,

'may find good reasons for ascribing the earth's form to the original fluidity of the mass in times long antecedent to the first introduction of living beings into the planet; but the geologist must be content to regard the earliest monuments which it is his task to interpret as belonging to a period when the crust had already acquired great solidity and thickness, probably as great as it now possesses, and when volcanic rocks not essentially differing from those now produced, were formed from time to time, the intensity of volcanic heat being neither greater nor less than it is now.'

There can be no doubt that astronomers have been startled into something like general protest against the rigid uniformitarianism of Sir Charles Lyell. Differing as they do very widely in their conceptions of the probable manner in which planets are formed, they seem to agree that those bodies have their beginning in heat and in fusion. The phenomena of variable stars, taken in connection with the revelations of spectrum analysis, demonstrate that the combustion and the cooling of starry masses are occurrences not unknown in the economy of the universe. If Sir Charles declines to contest the astronomical position of the original fluidity of the planet, considerable plausibility will continue to attach to that geological doctrine which connects the crystalline rocks with the fluidity in question. Those rocks, from the most ancient granites to the most recent clay-slates, occupy a large proportion of the earth's surface. Their great general antiquity is indisputable. The theory that they furnish the link between the past and the present of the earth's crust—that they furnish the point where the lights of geological and of astronomical science meet—strongly commends itself to the mind.

These observations derive additional force from the circumstance that Sir Charles Lyell's doctrine of the modern and chemical origin of all crystalline rocks is dependent upon considerations which must be allowed to possess not a little of a hypothetical and precarious character. The phenomena of metamorphism, as arising from heat, from thermal springs, and so on, are well-known and important; but there is nothing like adequate evidence that they are capable of giving the crystalline rocks that structure and aspect under which we behold them. The chemical substances in the crust which Sir Charles presumes to be capable of forming seas of molten matter, five miles deep and 5,000 miles long, have never placed before human eyes a lake of fire three miles across; is there not a trace of arbitrary hypothesis in supposing that, during hundreds of millions of years, those chemical agencies have been providing, beneath the surface of the world, cauldrons of fire to melt the granites of all known ages, from the Laurentian to the Tertiary, to produce the twistings, undulations, contortions of the metamorphic strata throughout hundreds of thousands of cubic miles of rock, and to feed every volcano that ever flamed on the planet? Not even to that proposition which is avowedly at the basis of Sir Charles's theory, namely, that the solidified shell of the earth is at least from 800 to 1,000 miles thick, can absolute certainty be said to belong. We are willing to admit the distinguished ability of Mr. Hopkins; but it is a fatal mistake to impute to solutions of problems in mixed mathematics that character of certainty which belongs to the results of purely mathematical reasoning. Into every problem of mixed mathematics one element at least enters which depends for its correctness upon observation. In many cases this correctness depends on the perfect accuracy of instruments, and upon consummate skill in using them. A minute error in the original observation may produce comprehensive error in the conclusion. It is still fresh in the public memory that new and more accurate observation corrected by millions of miles a calculation comparatively so simple as the distance between the earth and the sun. The problem by the solution of which Mr. Hopkins determined that the minimum thickness of the crust is from 800 to 1,000 miles depends for its reliability on certain obscure phenomena connected with precession and nutation. Sir Charles Lyell admits that the problem is a 'delicate' one. Mr. Charles MacLaren remarked, and Miller quotes the remark with approval, that Mr. Hopkins's inference 'is somewhat like an estimate, of the distance of the stars deduced from a difference of one or two seconds in their apparent position, a difference scarcely distinguishable from errors of observation.' Add to this that opinions might be quoted from mathematicians of name as decidedly in favour of the theory that the geological changes which have taken place in the earth's crust are due to central heat, as the deduction of Mr. Hopkins is opposed to it. In the ninth edition of his 'Principles,' i.e., in the edition immediately preceding that now current, Sir Charles informs us that

'Baron Fourier, after making a curious series of experiments on the cooling of incandescent bodies, considers it to be proved mathematically, that the actual distribution of heat in the earth's envelope is precisely that which would have taken place if the globe had been formed in a medium of a very high temperature, and had afterwards been constantly cooled.'

Sir Charles replied to this in the same edition that, if the earth were a fluid mass, a circulation would exist between centre and circumference, and solidification of the latter could not commence until the whole had been reduced to about the temperature of incipient fusion. We fail to see that this is an answer to Baron Fourier. What necessity is there for supposing that the solidification of the crust commenced before the matter of the globe had been reduced throughout to about the temperature of incipient fusion? The water in a pond must be reduced to about the temperature of incipient freezing before ice can form on the surface, but this does not prevent the formation of a sheet of ice on the top.

In the article in Nature, from which we have already quoted, Mr. David Forbes mentions that M. De Launay, Director of the Observatory at Paris, 'an authority equally eminent as a mathematician and an astronomer,' having carefully considered Mr. Hopkins's problem, decided that its data were incorrect, and that it could shed no light whatever on the question whether the globe is liquid or solid. There is some doubt, however, as to the import of M. De Launay's statement.

We may be the more disposed to wonder at the decision with which Sir Charles Lyell pronounces upon this subject in his latest edition, by the fact that, since the publication of the previous edition, he has modified, to a very serious extent, his conception of the evidence on which the theory which he adopts is based. In the ninth edition of the 'Principles' he laid so much stress on Sir Humphry Davy's hypothesis of an un-oxidized metallic nucleus of the globe, liable to be oxidized at any point of its periphery by the percolation of water, and thus to evolve heat sufficient to melt the adjacent rocks, that Hugh Miller, in contending against Sir Charles, selected this as an essential part of the argument. In his tenth edition Sir Charles does not even mention Sir Humphry Davy's theory. The star under the influence of which the tenth edition was prepared was that of Mr. Darwin. No brighter star may be above the geological horizon, and Sir Charles may have done well to own its influence, but we submit that opinions which undergo important modification within a few years ought hardly to be promulgated as marking the limit between the era of darkness and the era of light in geological discovery.

After all, however, the crucial question is, whether the theory of central heat has any positive evidence to support it. Here we meet, in the first place, with the undisputed fact that heat increases as we descend from the surface of the earth. Sir Charles Lyell admits that the fact of augmentation is proved. Experiment and observation, no doubt, have not yet enabled us to determine the ratio in which the heat increases as we penetrate into the crust; but this does not neutralise the force of the fact itself. Sir Charles endeavours to parry its effect by remarking that if we take a certain ratio of increase, a ratio which seems to be countenanced by experiment, we shall, 'long before approaching the central nucleus,' arrive at a degree of heat so great 'that we cannot conceive the external crust to resist fusion.' It is surely a sufficient reply to this to say that our conceptions as to the consequences arising from an admitted fact can neither invalidate its evidence nor annul the obvious inferences from it. The reader of the 'Principles of Geology,' besides, who has been told by Sir Charles Lyell that the interposition of a few feet of scoriæ and pumice enables him to stand without inconvenience on molten lava, may be permitted to form a high estimate of the power of many miles of stratified and unstratified rock to resist fusion by the internal fires. Sooth to say, however, it will be time to consider an objection grounded on the ratio of the increase in heat from the surface of the earth downwards, when the ratio in question has been ascertained. The fact of increase is admitted; the ratio of increase is an unknown quantity: it is curious logic to impugn the direct bearing of the former, on the strength of consequences conceived to arise from the latter.