AND KIVER VALLEYS BOEDEEING THE BRITISH ISLES. 321 



of tlie water as indicated by the soundings, to map out the margins 

 of the British area before it became divided up into the existing 

 islands. The conclusions arrived at are not only interesting, but 

 add much to our knowledge of geographical evolution, as brought 

 about by natural causes during immense periods of time. 



The 100-fathom line, running parallel with the present coasts, 

 has been the chief datum which former observers have taken for 

 their hydro -geographic workings ; but, as he points out, that line 

 is not everywhere coincident with the margin of the old plateau 

 from which our islands now rise ; for, especially in one tract west 

 of Ireland, this " British Platform," as it is called, extends further 

 out for 280 miles ; and, again, to the north and north-west it forms 

 part of the great Icelandic plateau, stretching to Greenland and 

 N'orth America. 



Between the Hebrides and Rockall a deep gulf (of 2,000 fathoms) 

 extends northward to about 50° I^. latitude, opening out southward, 

 at about 55° N". latitude, into the open ocean. The cliffs must have 

 been successive steps of enormous escarpments, of rapid descent, 

 except at the head of the gulf just mentioned, and of a smaller 

 gulf cutting into the " British Platform " south-west of Ireland. 

 The old Shannon entered the head of this bay, and the " Porcupine 

 Bank " was the most elevated part of that portion of the platform 

 which was cut off by the western gulf. 



The river which brought the Seine and other streams down the 

 valley, since changed into the English Channel, had deep canons 

 in its course (indicated by deeper local soundings) ; and, like the 

 old Irish-Channel river and the Erne, higher up on the Irish coast, 

 must have fallen into the sea with grand waterfalls, if seen from 

 the ocean. The Shannon, however, opening into the shallow head 

 waters of the above-mentioned bay south-east of the " Porcupine 

 Bank," emptied itself less precipitately. 



The distribution of animals and plants supports the conclusions 

 arrived at by the author, as well as the relative periods at which 

 the fauna and flora were distributed, and afterwards interfered with 

 by the elevation of the land to the height of 7,000 feet. This 

 change of level, as indicated in the author's former memoir, lately 

 read before this Institute, brought about the greatest intensity 

 of cold in the so-called Glacial Period. This has, fortunately 

 for us, been modified, chiefly by the subsidence of the land to its 

 present height. 



The features of the ancient coast-line of this region are com- 

 pared with those of the North American area, as defined by 

 Dr. Spencer and others. 



Reference is made to observations by others on the possible 

 down-sinking of ocean- waters and the uprising of lands ; and the 

 author reminds us that we need not be surprised at the great 

 movements in Pliocene times, for the present difference between 

 the highest land and the lowest sea-bed is not quite 60,000 feet, 



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