112 THE POPULAR SCIENCE MONTHLY. 



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 pe- 

 riods are indulged in," says Sir C. Lyell, " the imagination is apt to 

 take alarm at the immensity of the time required to interpret the monu- 

 ments 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 modi- 

 fied the animate and the inanimate world with extraordinary and exces- 

 sive 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 begining 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 Ave 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 thick- 

 ness, and many which there measure only a few inches, on the Conti- 

 nent expand into strata of immense depth ; while others, of great 

 impoi'tance 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) else- 

 where, 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 pro- 

 duced 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. 



