226 SUPEKFICIAL DEPOSITS. 



Mr. Collenette* makes an ingenious suggestion as to the 

 origin of our clay. He thinks, as I do, that these islands 

 shared in the wide-spread subsidence which took place during 

 the glacial period, and that following this submergence came 

 a time of elevation and glaciation, during which, under the 

 action of cold, the decomposed and weathered rocks broke up 

 and formed rubble and clay. " Then," to quote his own 

 words, " came a period of comparative heat, with great melt- 

 ing of the ice in successive summers, and re-glaciation in 

 successive winters, washing down clay and sand from the de- 

 composed rock, filling up hollows and the interstices of the 

 rubble of the cliffs, spreading it out over the planed rocks, 

 filling up the lower valleys, in fact, using an immense mass of 

 decomposed material left on the top of the island by the sea 

 to cover all the lower levels with sand and clay." 



But the wash from glaciers could not well have laid down 

 the clay on the plateau, the high table land of the islands, nor 

 could it have deposited the bed covering the raised beach on 

 the top of South Hill, for the Mont de la Ville, of which 

 South Hill forms part, is an isolated hill, or detached piece of 

 the table land, and out of the reach of any such flow. Again, 

 it does not seem likely that the deposit on La Motte Island 

 could be due to this cause. Had the island been opposite the 

 mouth of a valley, far as it is from the hill sides, it might have 

 yet been just possible, but it lies off a long stretch of low-lying 

 land. The presence of beds of rolled stones in the clay seems 

 to me also to be an objection to this theory, and to that of 

 Prestwich. 



I do not feel prepared to add another to the many theories 

 already put forward as to the origin of the rubble drift and 

 its congeners, but perhaps I may venture to make a suggestion 

 with regard to the yellow clay of our islands. 



During the early and coldest part of the glacial period, 

 and probably for ages before, Jersey and Guernsey stood at a 

 higher level than they do now, possibly forming part of 

 the mainland. During all this time the rocks were being 

 weathered and disintegrated, the process being intensified 

 when the cold grew severe by the action of frost and ice, so 

 that the surface, latterly greatly denuded of vegetation, be- 

 came covered with loam and rock fragments. The islands 

 then shared in a submergence! affecting at least the greater 

 part of the British Islands and the north of France, and which 



* Presidential Address. Guernsey Society of Natural Science, 1895. 



t In my paper on the Raised Beaches of Jersey, by an oversight in correcting 

 the proof, I am made to say that I believe the submergence to have taken place in 

 late post-glacial times ; it should have been late glacial times. 



