2 6o POPULAR SCIENCE MONTHLY. 



eluding in the estimate the time in which the cold was increasing, 

 or Preglacial time, and that during which it was diminishing, or 

 Postglacial time. Details were given to show that the estimate of 

 one foot on an average being removed from the surface by denuda- 

 tion in six thousand years, on which estimate the hypothesis that 

 eighty thousand years had elapsed since the Glacial epoch was 

 founded, was insufficient, as a somewhat heavier rainfall and the 

 disintegrating effects of frost would produce far more rapid denuda- 

 tion. It was incredible that man should have remained physically 

 unchanged during so long a period. At the same time, evidence 

 that had been brought forward of the occurrence of human relics 

 in Preglacial times had led the author to change his views as to the 

 age of the high-level gravels in the Somme, Seine, Thames, and 

 Avon Valleys, and he was now disposed to assign these beds to the 

 early part of the Glacial epoch, when the ice sheet was advancing. 

 This advance drove the men who then inhabited western Europe to 

 localities which were not covered with ice. Man must, however, 

 have occupied the country but a short time before the land was 

 overwhelmed by the ice sheet. The close of the Glacial epoch, or 

 the final melting of the ice sheet, might have taken place between 

 eight and ten thousand years ago. 



His latest and most matured views on this subject, agreeing sub- 

 stantially with these expressions, were embodied in his book, Geol- 

 ogy: Chemical, Physical, and Practical, which was published in 

 1886 and 1888. In these volumes he gave more positive expression to 

 the view which had been for some time assuming shape with geolo- 

 gists, and the acceptance of which had been made imperative by the 

 results of geological surveys in America, that, while the great time 

 divisions of geology may be of almost universal application, the 

 smaller breaks in continuity, which are of frequent occurrence in all 

 areas, are subject to constant differences of extent and value. Con- 

 sequently, in filling up the details of the several geographical areas, 

 each one is found to have its own local stamp, and possesses its own 

 special terms, some knowledge of which is as essential to the geolo- 

 gist as the language of a country is to the traveler, if he would pass 

 through it with profit. 



In the preface to this work Professor Prestwich gave a clearer 

 definition of his attitude toward the different schools of geological 

 thought, observing that the doctrine of non-uniformity must not be 

 confounded with reliance on catastrophes, and that it does not involve 

 any questions respecting uniformity of law, but only those respecting 

 uniformity of action. "I myself," he says, "have long been led 

 to conclude that the phenomena of geology, so far from showing 

 uniformity of action in all time, present an unceasing series of 



