RIVEE VALLEYS OF THE WEST AFRICAN CONTINENT, ETC. 163 



COMMUNICATION" RECEIVED. 



Professor J, Logan Lobley, F.G.S., writes : — 

 All geologists ouglit to be gi^ateful to Professor Hull for con- 

 tinuing his investigations of the ocean floor along the eastern side 

 of the Atlantic. The results of these investigations are geo- 

 graphicallj highly interesting, but geologically they are more 

 than interesting, they are exceedingly valuable. 



Apart from the submarine ravines revealed, the continuation of 

 the continental platform from Iceland in the north, to the south of 

 the equator, is a most important fact, from which only one con- 

 clusion can be drawn — that there has been a general subsidence of 

 the western parts of Europe and Africa from a former higher 

 level. The suggestion that this great terrace, at the summit of a 

 steep ascent of thousands of feet, and, with the exception of far 

 separated indentations, continuous for thousands of miles, has 

 been formed by the deposition of material at each side of the 

 mouths of existing rivers that have brought it from the interior 

 lands, is altogether untenable, for it will not for a moment bear 

 examination. 



If this be so, the formation of the depressions ci'ossing the 

 terrace has cibviously been by subaerial water erosion, and the 

 position of these depressions with respect to present river valleys 

 undoubtedly points to the former continuation seawards of these 

 valleys and their excavating rivers. 



Thegreat depth of the submarine ravines, however, if the whole 

 has been due to subaerial erosion, requires such a great eleva- 

 tion of enoi'mous areas of land in late geological times, that some 

 hesitate to accept this conclusion. But other evidences of great 

 changes of level in Post Tertiary times, the consideration that 

 5,000 feet is but the -soVo P^^'^ o^ ^^^ diameter of the globe, and 

 that such an elevation affords the most simple explanation of the 

 cause of the Glacial epoch, ought to lessen the disinclination to 

 accept conclusions which have been drawn by Professor Spencer, 

 Mr. Warren Upham, and Professor Hull, from the phenomena of 

 the Atlantic floor both in its western and its eastern margins. 



One apparent inconsistency is the greatness of the submarine 

 ravine attributed to the lesser River Adour and the smallness of 

 that attributgd to the greater River Gironde. This, it seems to 

 me, admits of a complete explanation, as I endeavoured to show to 



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