516 S. H. Roxcorth — Tracer of a Great Tod-Glacial Flood. 



accumulations of winter snow may, on sloping surfaces, have tended 

 in a lesser degree to the same result ; or, thirdly, the transient passage 

 of a body of water may have siuept the land debris down to the shore- 

 line and to a certain distance beyond. With regard to the action of 

 ice, I feel about it the difficulty before expressed on the score of the 

 presence of organic remains, and also although a local mass of ice 

 might have radiated from the higher centi'al ground southward 

 towards the Bill, and in a north-westerly direction at the other end 

 of the island, the size of Portland and its height are too limited for 

 local glaciers ; while if it formed part of a continental ice-sheet, it 

 would have a more definite and uniform direction, and we should 

 expect to find on the adjacent mainland traces of that ice-sheet, 

 which we do not. The limited size of the island forms also a 

 similar but not so strong objection to the operation of accumulated - 

 snow. I find in the third alternative an explanation which agrees 

 with the observed conditions. Sir E. I. Murchison, with respect to the 

 Brighton cliff, and myself with respect to the Sangatte cliff, had, so 

 far back as 1851, arrived independently at somewhat similar con- 

 clusions regarding the conditions under which they were formed, 

 save only that his generalizations were more extreme, attributing to 

 one operation what I should attribute to several. He supposed that 

 the angular debris resulting from an extended catastrophe caused by 

 great waves produced by great oscillation and violent fractures of 

 the crust of the earth, ' by which the earth's surface has been so 

 powerfully affected in former times,' passing over the land, and by 

 ' the large area under consideration being suddenly broken up and 

 submerged.' I then limited my conclusions to the fact that the 

 accumulation of the Sangatte drift was tumultuous and of short 

 duration, and that it was formed under water." 



A third very opportinie paper for my purpose, also by Mr. 

 Prestwich, was published by him in the May number of the 

 Geological Journal, p. 127, etc., "On a Peculiar Bed of Angular 

 Drift in the Lower-Chalk High Plain between Upton and Chilton." 

 In this paper he describes a considerable deposit of angular drift 

 containing Mammalian remains and land-shells and angular pieces 

 of chalk. " The bones," he says, " were mostly in a broken and 

 fragmentary state, but though broken they are not rolled or water- 

 worn." . . . "All the fragments of flint, even the smallest (and they 

 are few more than two or three inches in length), are perfectly sharp 

 and angular, and are generally not discoloured." Mr. Prestwich 

 says of the deposit : " There is no trace in any part of this section 

 of stream or river action, .... the shells consist essentially of land 

 shells ; the two exceptions are so rare that they only prove the 

 rule ; for Planorbis albus, which lives on water plants and frequents 

 marshes, can pass over land surfaces, while the Limnoea truncatula is 

 nearly amphibious, being more frequently met with out of the water 

 than in it ; it is also found in very elevated spots." 



The peculiar features of this drift are enumerated by Mr. Prestwich, 

 as the absence of worn debris indicating prolonged water action, and 

 of either marine or fluviatile remains ; the presence, it may be said, 



