one, would, at an interval of 10,000 years, render them suitable for the other. That man existed in Europe during the period of the mammoth, no longer, apparently, admits of a doubt. "When speculations on the long series of events which occurred in the glacial and post glacial periods are indulged in," says Sir C. Lyell, "the imagination is apt to take alarm at the immensity of the time required to interpret the monuments of these ages, all referable to the era of existing species. In order to abridge the number of centuries which would otherwise be indispensable, a disposition is shown by many to magnify the rate of change in prehistoric times, by investing the causes which have modified the animate and the inanimate world with extraordinary and excessive energy.... We of the living generation, when called upon to make grants of thousands of centuries in order to explain the events of what is called the modern period, shrink naturally at first from making what seems so lavish an expenditure of past time."
To the geologist, however, these large figures have no appearance of improbability. All the facts of geology tend to indicate an antiquity of which we are but beginning to form a dim idea. Take, for instance, one single formation—our well known chalk. This consists entirely of shells and fragments of shells deposited at the bottom of an ancient sea, far away from any continent. Such a progress as this must be very slow: probably we should be much above the mark if we were to assume a rate of deposition of ten inches in a century. Now the chalk is more than a thousand feet in thickness, and would have required, therefore, more than 120,000 years for its formation. The fossiliferous beds of Great Britain, as a whole, are more than 70,000 feet in thickness, and many which there measure only a few inches, on the Continent expand into strata of immense depth; while others, of great importance elsewhere, are wholly wanting there, for it is evident that, during all the different periods in which Great Britain has been dry land, strata have been forming (as is, for example, the case now) elsewhere, and not with us. Moreover, we must remember that many of the strata now existing have been formed at the expense of older ones; thus, all the flint gravels in the southeast of England have been produced by the destruction of chalk. This, again, is a very slow process. It has been estimated that a cliff 500 feet high will be worn away at the rate of an inch in a century. This may seem a low rate, but we must bear in mind that along any line of coast there are comparatively few points which are suffering at one time, and that even on those, when a fall of cliff has taken place, the fragments serve as a protection to the coast, until they have been gradually removed by the waves The Wealden Valley is 22 miles in breadth, and on these data it has been calculated that the denudation of the Weald must have required more than 150,000,000 of years.
