Hi PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



in a year on a surface of 3000 miles ; and therefore 1000 feet of 

 "Wealden beds might thus be deposited in 12,000 years. 



Again, the Weald of Sussex has been denuded by watery action, 

 and its arch of marine and fluviatile strata cut down on an average 

 about 1100 feet. Supposing the denudation to have been by at- 

 mospheric and river action, and at the same rate as the waste of 

 surface in the Gangetic area, we shall find it necessary to give 

 12000 x 111=1332000 years for the effect*. 



If it be supposed to have been accomplished by the sea cutting 

 its way against cliffs, we may, by assuming this waste equal to the 

 most rapid destruction of any considerable part of our coast (2\ yards 

 in a year by measure on the Holderness coast), considerably reduce 

 the period. But Mr. Darwin, by assuming a very slow rate of waste 

 (1 inch in a century), augments the time of this operation to the 

 inconceivable number of 306,662,400 years f ! To show how little 

 these computations are relied on, it is enough to say that Sir E. I. 

 Murchison does not admit the basis of either, denying the denuda- 

 tion to have been by ordinary atmospheric or ordinary oceanic 

 agencies. 



Do not geologists sometimes speak with heedless freedom of the 

 ages that have gone ? Such expressions as that " time costs Nature 

 nothing" appear to me no better than the phrase which ascribes to 

 Nature " the horror of a vacuum." Are we to regard as information 

 of value the assertion that millions on millions of ages have passed 

 since the epoch of life in some of the earlier strata ? Is not this 

 abuse of arithmetic likely to lead to a low estimate of the evidence 

 in support of such random conclusions, and of the uncritical judg- 

 ment which so readily accepts them ? 



Dismissing, then, any further examples of this geological calculus, 

 I may call your attention to one case of recent inquiry bearing on 

 the connexion of geological with historical time. 



The River Somme, of historical celebrity, rising near St. Quentin, 

 and flowing by the walls of Peronne and the towers of Amiens, occu- 

 pies a gentle valley, not 100 miles in the whole length, in the chalk 

 country of Picardy. For a considerable part of this course the valley 

 discloses at intervals deposits of gravel sometimes elevated as much 

 as 100 feet above the river, and 6 to 12 feet thick, covered by whitish 

 marls and sand and unstratified brick-earth. The gravel is irregularly 

 stratified with sand ; it consists mainly of small fragments of flint not 

 much worn by attrition, but encloses besides masses of tertiary sand- 

 stone comparable to the " greywethers " of Wiltshire, and wedge- 

 shaped masses of flint several inches long, which appear to have been 

 shaped by art, for purposes of digging earth or scraping wood, or 

 less peaceful occupations. The gravel- deposit appears to be of fluvia- 



* Within the drainage of the Granges (300,000 square miles) the average waste 

 of the whole surface appears, by the amount of impurity in its waters, to be 

 yiyth of an inch annually. 



t In this computation there seems an error in principle — viz. that it:costs 500 

 times as long a period to waste a cliff 500 times as high. Cliffs are not wasted in 

 inverse proportion to their height. 



