the conveyance of the materials of the strata to the points of their deposition, they are in the main excluded from the question. There are no traces of their influence until a large part of the depositions were formed, and whatever effects they may have produced towards the close of the secondary and during the tertiary periods, they must, from the resistance currents from the land meet on entering the ocean, have been confined to the vicinity of the points of their debouchure. The question, therefore, as far as all the most extensive and important effects are concerned, relates only to the chemical and volcanic forces, and the mechanical agencies of the ocean under which the strata were formed, and subsequently thrown into their present conditions.

And in the first place, the assumption that all these great effects are the result of the causes that are now in activity, and arose gradually from agencies of essentially the intensity they are now exerting, is based on altogether inadequate and mistaken ground. It certainly is not a self-evident proposition. There is nothing in the nature of the strata themselves that shows directly that they must have been formed exclusively by causes like these, acting with only their present energy. If that is made a principle or postulate of the science, it must first, like other facts, be established by appropriate evidence. But no such demonstration of it is furnished by geologists.

It is either assumed without any attempt at its verification, or founded on vague and imaginary analogies. Thus Sir Charles Lyell, for example, who in the construction and support of his theory reasons altogether from the present to the past, takes for granted at every step of his argument, the point on which its validity depends;* namely, not only that the causes now producing geological effects on the globe are the same in kind as those to which the stratified rocks owe their existence and modifications, but that the scale on which they are now acting, and the rate at which they are giving birth to their several effects, are the measure of the energy with

* "Now the principal source from whence we are enabled to draw such conclusions respecting the nature of the solid materials of the earth, and the changes which they have undergone, is a comparison of geological phenomena with the effects previously known to have been produced in modern times by running water and subterranean heat. Hence the utility of one of the preceding treatises on aqueous and igneous causes, in which it was shown that strata are at present in the course of formation by rivers and marine currents; both in seas and lakes; and that in several parts of the world rocks have been rent, tilted, and broken, by sudden earthquakes; or have been heaved up above, or let down below their former level; also that volcanic eruptions have given rise to mountain masses made up of scoriæ, and of stone both porous and solid.

"From these remarks it will be seen that a study of systematic treatises on the recent changes of the organic and inorganic world, affords a good preliminary exercise for those who desire to interpret geological movements. They are thus enabled to proceed from the known to the unknown, or from the observed effects of causes now in action to the analogous effects of the SAME or SIMILAR CAUSES which have acted at remote periods."-Lyell's Principles, Preface, pp. xiii., xiv.

which they acted, and the rapidity with which they wrought their results, in the formation of the strata. For he offers nothing but the effects themselves that are now in the process of production, the strength of the agents that are bringing them into existence, and the rapidity with which they are wrought as proofs that all the geological effects of ancient times which it is his aim to explain, were wrought, by the same agents at the same rate; and thence makes the ground of his inference that the periods occupied in their production must have been of the immeasurable length which he ascribes to them. But the effects that are now taking place plainly yield no verification of his inference, unless it is either self-evident, or is shown by extraneous proof, that all the geological effects in question must necessarily have been produced by the same cause, acting, uniformly with the same energy. But that, instead of proving, he takes for granted. His argument, accordingly, expressed syllogistically, is nothing more than the following:

All the geological changes that have been produced on the globe have been the work of causes identically the same in kind, energy, and the rapidity with which they produced their effects.

But the causes that are now giving birth to geological changes are feeble, and advance at a very slow rate in the production of their effects.

Therefore, the causes under which the formation of the stratified rocks took place, must have been similarly feeble, and advanced at a similarly slow rate in the production of their effects.

And again:

The energy with which geological causes act, and the rate at which they give birth to their effects, are uniformly the same at all periods.

But the energy of the causes that are now working changes on the earth is slight, and long periods are occupied in the completion of their effects.

Therefore, the causes by which the strata of the earth were produced were equally slight in their energy, and periods equally long in proportion to the magnitude of the effects they produced, were occupied in their completion.

The whole point to be established, is thus assumed in the premise from which it is deduced. He proceeds throughout his discussion on a mistaken view of the real question in debate-which is, what the causes were by which the stratified rocks were formed, and what the mode was of their agency and the rapidity with which they wrought their effectswhich is to be determined by the nature of the effects; not—which is the position he employs himself in endeavoring to evince-whether, on the supposition that the causes that are giving birth to geological effects, are in nature, strength, and rate of produc

His

tion, identically like those by which all former effects were produced, immeasurably long periods must not have been occupied in their completion. In the one case, those ancient effects are made the measure of their causes; in the other, modern effects, which are wholly inferior in magnitude, and in a large degree of a different nature, are made their measure. whole system is thus built on the assumption of the premise from which it is deduced. He accordingly does not generally attempt directly and absolutely to demonstrate the solutions he suggests of the phenomena, real or presumed, which he endeavors to explain; but presents them simply as suppositions which admitting the postulate on which he proceeds -furnish possible or probable explanations of them. Thus it is by such a mere hypothesis that he endeavors to account for the great variations in the temperature of the globe, which he assumes have taken place.

"I shall now proceed to speculate on the vicissitudes of climate which must attend those endless variations in the geographical features of our planet, which are contemplated in geology. That our speculations may be confined within the strict limits of analogy, I shall assume, 1st, That the proportion of dry land to sea continues always the same; 2dly, That the volume of the land rising above the level of the sea is a constant quantity; and not only that its mean,