Illustrations of the Huttonian Theory of the Earth

by John Playfair

447. This unfavourable view of geology, ought not, however, to be received without examination; in science, presumption is less hurtful than despair, and inactivity is more dangerous than error.

One reason of the rapid succession of geological theories, is the mistake that has been made as to their object, and the folly of attempting to explain by them the first origin of things. This mistake has led to fanciful speculations that had nothing but their novelty to recommend them, and which, when that charm had ceased, were rejected as mere suppositions, incapable of proof. But if it is once settled, that a theory of the earth ought to have no other aim but to discover the laws that regulate the changes on the surface, or in the interior of the globe, the subject is brought within the sphere either of observation or analogy; and there is no reason to suppose, that man, who has numbered the stars, and measured their forces, shall ultimately prove unequal to this investigation.

448. Again, theories that have a rational object, though they be false or imperfect in their principles, are for the most part approximations to the truth, suited to the information at the time when they were proposed. They are steps, therefore, in the advancement of knowledge, and are terms of a series that must end when the real laws of nature are discovered. It is, on this account, rash to conclude, that in the revolutions of science, what has happened must continue to happen, and because systems have changed rapidly in time past, that they must necessarily do so in time to come.

He who would have reasoned so, and who had seen the ancient physical systems, at first all rivals to one another, and then swallowed up by the Aristotelian; the Aristotelian physics giving way to those of Des Cartes; and the physics of Des Cartes to those of Newton; would have predicted that these last were also, in their turn, to give place to the philosophy of some later period. This is, however, a conclusion that hardly any one will now be bold enough to maintain, after a hundred years of the most scrupulous examination have done nothing but add to the evidence of the Newtonian System. It seems certain, therefore, that the rise and fall of theories in times past, does not argue, that the same will happen in the time that is to come.

449. The multifarious and extremely diversified object of geological researches, does, no doubt, render the first steps difficult, and may very well account for the instability hitherto observed in such theories; but the very same thing gives reason for expecting a very high degree of certainty to be ultimately attained in these inquiries.

Where the phenomena are few and simple, there may be several different theories that will explain them in a manner equally satisfactory; and in such cases, the true and the false hypotheses are not easily distinguished from one another. When, on the other hand, the phenomena are greatly varied, the probability is, that among them, some of those instantiæ crucis will be found, that exclude every hypothesis but one, and reduce the explanation given to the highest degree of certainty. It was thus, when the phenomena of the heavens were but imperfectly known, and were confined to a few general and simple facts, that the Philolaic could claim no preference to the Ptolemaic system: The former seemed a possible hypothesis; but as it performed nothing that the other did not perform, and was inconsistent with some of our most natural prejudices, it had but few adherents. The invention of the telescope, and the use of more accurate instruments, by multiplying and diversifying the facts, established its credit; and when not only the general laws, but also the inequalities, and disturbances of the planetary motions were understood, all physical hypotheses vanished, like phantoms, before the philosophy of Newton. Hence the number, the variety, and even the complication of facts, contribute ultimately to separate truth from falsehood; and the same causes which, in any case, render the first attempts toward a theory difficult, make the final success of such attempts just so much the more probable.

This maxim, however, though a general encouragement to the prosecution of geological inquiries, does not amount to a proof that we are yet arrived at the period when those inquiries may safely assume the form of a theory. But that we are arrived at such a period, appears clear from other circumstances.

450. It cannot be denied, that a great multitude of facts, respecting the mineral kingdom, are now known with considerable precision; and that the many diligent and skilful observers, who have arisen in the course of the last thirty years, have produced a great change in the state of geological knowledge. It is unnecessary to enumerate them all; Ferber, Bergman, De Luc, Saussure, Dolomieu, are those on whom Dr Hutton chiefly relied; and it is on their observations and his own that his system is founded. If it be said, that only a small part of the earth's surface has yet been surveyed, and described with such accuracy as is found in the writers just named, it may be answered, that the earth is constructed with such a degree of uniformity, that a tract of no very large extent may afford instances of all the leading facts that we can ever observe in the mineral kingdom. The variety of geological appearances which a traveller meets with, is not at all in proportion to the extent of country he traverses; and if he take in a portion of land sufficient to include primitive and secondary strata, together with mountains, rivers, and plains, and unstratified bodies in veins and in masses, though it be not a very large part of the earth's surface, he may find examples of all the most important facts in the history of fossils. Though the labours of mineralogists have embraced but a small part of the globe, they may therefore have comprehended a very large proportion of the phenomena which it exhibits; and hence a presumption arises, that the outlines, at least, of geology have now been traced with tolerable truth, and are not susceptible of great variation.

451. When the phenomena of any class are in general ambiguous, and admit of being explained by different or even opposite theories; if few of those exclusive facts are known, which admit but of one or a few solutions, then we have no right to expect much from our endeavours to generalise, except the knowledge of the points where our information is most deficient, and to which our observations ought chiefly to be directed. But that many of the exclusive and unambiguous instances are known, in the natural history of the globe, I think is evident from the reasoning in the foregoing pages, where so many examples have occurred of appearances that give the most direct negative to the Neptunian system, and exclude it from the number of possible hypotheses, by which the phenomena of geology can be explained. The abundance of such instances is an infallible sign, that the mass of knowledge is in that state of fermentation, from which the true theory may be expected to emerge.

452. Another indication of the same kind, is the near approach that even the most opposite theories make, in some respects, to one another. There are so many points of contact between them, that they appear to approximate to an ultimate state, in which, however unwillingly, they must at last coincide. That ultimate form, too, which all these theories have a tendency to put on, if I am not deceived, is no other than that of the Huttonian theory.