406 



PROF. J. PKESTWICH 0]ST THE GLACIAL PEEIOD, "WITH 



this district now varies from 65 inches on the coast to 42 inches in 

 the interior ; but during the Glacial period, Dana considers that the 

 special conditions must have occasioned a much more abundant pre- 

 cipitation — probably as high as 120 inches, or greater than in any 

 modern glacial region. 



In this country and in the north of France the valleys have been 

 excavated to the depth of from 80 to 120 feet in glacial and postglacial 

 times. It may be difficult from our present experience to conceive 

 this to have been effected in a comparatively short geological time, 

 though the extension back to glacial time gives both greater power 

 and greater time ; yet it is equally, and to my mind more, difficult 

 to suppose that Man could have existed 80,000. years (or 200,000, 

 if Preglacial), and that existing forms of our fauna and flora should 

 have survived during 240,000 years without change and modification. 

 The acceptance of those dates, which place the land-glaciation some 

 100,000 to 150,000 years back, would also lead to the difficulty (even 

 on the assumption of a rate of denudation of 1 foot in 6000 years)* 

 that the surface-wear should have been far greater than it is. 

 For example, to mention only two points, could the striations on 

 soluble rock-surfaces have remained so fresh as they are, and would 

 not the limestone-rock on which the boulders of Silurian rocks 

 were left on the melting of the ice on the Yorkshire hills show 

 much greater wear than it actually does ? These boulders now stand 

 on pedestals raised from 1 to 2 feet above the surrounding surface-level 

 in consequence of the dissolving away of the limestone rocks. We 

 should look for pedestals of much greater height if the glaciation 

 took place at the distant period involved in Dr. Croll's hypo- 

 thesis. 



My first impressions with respect to theYalley of the Sommewere: — 

 that the high-level gravels originated in early Glacial times ; that the 

 intermediate stages and terraces were formed during the excavation 

 of the valley as a consequence of the great glacial and postglacial 

 floods ; and that the low-level gravels formed the concluding stage of 

 those conditions. But in the absence of data, since acquired, the 

 strong prepossessions then existing, and the novelty of the subject, 

 I was then led to conclude that the whole might be Postglacial. 



So much evidence has, however, since been brought forward with 

 respect to the so-called Preglacial Man, that I feel I am now justi- 

 fied in reverting in great part to my original position. The cave- 

 work of Mr. Tiddeman and Dr. Hicks gives strong presumptive evi- 

 dence of the earlier geological appearance of Man in the British area ; 

 and I see no reason to doubt the sub-boulder-clay evidence of Mr. 

 Skertchly, although I was unable myself to corroborate his discovery 

 of the worked flints. Of the correctness of his opinion in respect to 

 the stratigraphical position of the bed in which his specimen was 

 found, I have, however, little doubt. The great masses of gravel in 

 the neighbourhood of Mildenhall and Lakenheath, also containing 

 flint implements, are certainly not of fluviatile origin ; they seem 



* A general rate of this description is also scarcely applicable to a special 

 rate, such as that relating to valley-denudation. 



