228 Pi'of. Hughes, Criticism of the Geologicat [Oct. 30, 



just quoted. The thickness of the strata thus folded is very great. 

 The Mesozoic and Tertiary strata alone have been estimated at 

 more than 50,000 feet\ 



One controverted point in connection with this question of the 

 periodic oscillation of all known land areas is the inference that 

 these movements have been continued down to quite recent times. 

 Heim has recently shown that the up-valley slope of some 

 terraces along the Lake of Zurich indicates that the hollow in 

 which the lake lies holds the water in consequence of a depression 

 of the region in post-glacial times. But the strongest argument 

 in favour of this is derived from the occurrence of marine de- 

 posits of post-glacial age at various levels up to at least 1850 

 feet, on the flanks of the hills in Britain and Scandinavia. The 

 shell beds of Uddevalla and the fossiliferous sands and clays of 

 Macclesfield and of North Wales are the most important ex- 

 amples. I have already elsewhere described those of North 

 Wales^. It has been generally recognised that the occurrence 

 of marine terraces at various levels along the marginal moun- 

 tain regions would be a strong argument in favour of eleva- 

 tion of the land having had much to do with the occurrence of 

 glaciers in that region. At any rate the proof thus offered of 

 earth movements to that extent in post-glacial times would be a 

 complete answer to the objection that the advocates of the geo- 

 graphical theory were calling in hypothetical changes on the 

 earth's surface of which they had no proof The extreme glacialists 

 therefore received with great favour the view that these marine 

 deposits did not represent old beaches, but were the sweepings of 

 the sea bottom pushed by the advancing ice up the hill sides, and 

 left there when it receded. The objection that the beds were 

 distinctly stratified and the material sorted by water was met by 

 the suggestion that it was rearranged by the fresh water which 

 flowed from the end of the melting ice. 



But an examination of the beds renders this explanation 

 extremely improbable. The drift is not thrown out in cones of 

 dejection and fan-shaped masses, nor arranged in terraces along 

 the possible course of glacier-born streams, but occurs in the bays 

 where it was thrown up by the sea and protected afterwards from 

 destruction, or lies in deep valleys which during the submergence 

 were invaded by the tides. The boulders and shells are thrown 

 ashore and buried in the slips of boulder clay cliffs and the waste 

 of older marine deposits. Sometimes the scratched stones from 

 these boulder clay cliffs are not washed far enough to lose their 

 strise but, carefully packed in clay, have been preserved unworn. 



^ Judd, Volcanoes, p. 295. 



2 "On the Drifts of the Vale of Clwyd and their Relation to the Caves and Cave 

 Deposits," g. J. G. S., Nov. 17, 1886, Vol. xliii. p. 73. 



