will wholly disappear in four or five million years. Geology teaches us that the history of the crust of the earth embraces a period infinitely longer than this. That is enough to prove that another factor does intervene—that is, manifestations of internal energy, which disturb from time to time the acquired states of equilibrium, and restore a new force to the decreasing outer powers. Thus, instead of assuming a regular process of planing down, I laid down in principle that things would not go on in this way. A geologist, besides, could not reason otherwise, except he mistook the daily teachings of science which show him at every instant foldings and contortions of strata, certain signs of an order of things very different from the regular pursuit of external influences.
"Seeking, then, to place myself under the most unfavorable conditions for my theory, I supposed the ancient history of the globe divided into tranquil periods, each of four or five million years, separated from one another by so many ruptures of equilibrium. How many of these periods would be required to account for all the known sedimentary formations? In trying to solve this problem, I remarked that each period would have cast into the ocean a cube of débris which, scattered, according to Mr. J. Murray, through only one fifth of its area (the fraction over which, according to the soundings, the sedimentation of detritus extends), would form a bed of some six or seven hundred metres in mean thickness. It seemed to me reasonable to suppose that this thickness, null at the extreme further side of the deposits, would increase slowly at first, and then more rapidly toward the neighborhood of the coast, where it might attain a maximum of two kilometres. Dana having estimated at forty-five thousand metres the united depth of all the sedimentary formations when each is measured at its point of greatest thickness, I drew the conclusion that all geological history could be included within a time certainly less than ninety million years. After the publication of this note I was called upon by the general secretary of the Geographical Society to supply the place of a speaker who was unable to fulfill his engagement. I responded, undertaking to call the attention to the phenomena of erosion which I had been studying, without elaborating the purely geological considerations. I have recently presented an extended paper on these details to the Catholic International Scientific Congress, and in it have examined all the phases of the question. This paper will be published in full, and in it M. Ldotard will be able to learn how greatly my views differ from those which he has mistakenly attributed to me.
"I will here only correct a grave error which my critic commits when he charges me with not having taken account of the accrement which volcanic action brings to the dry land. M. Léotard
