312 PEOr. HULL^ LL.D., F.E.S,^ ON THE SUBMERGED TERRACES 



below the general floor of the sea bed.* Here, we may sup- 

 pose, the channel has been kept open and free fi'om sedi- 

 ment, unlike the portions of the river valley above and below. 

 The cause of this dissimilarity of conditions is not far to seek. 

 On looking at the map it will be seen that the " Hurd Deep" 

 lies in the narrowest part of the channel west of the Straits 

 of Dover, between the Isle of Wight and Portland Bill on 

 the north, and Cape de la Hague and Cape de Barfleur on the 

 south. Above and below this strait the channel broadens 

 out to about twice its breadth between these points ; hence 

 the tidal currents have here extraordinary force and swift- 

 ness, owing to which the sediment, deposited above and 

 below, appears to have been prevented from settling down 

 and filling up the gorge of the old river. The general out- 

 line, the direction, and position of this remarkable rift, all 

 point to the " Hurd Deep " as a river channel which has been 

 cut down into the solid rock, and is bounded by steep, or 

 precipitous, cliffs resembling on a small scale the American 

 canons. The two submarine rivers here described must have 

 exceeded in size any of our existing streams, and we may 

 infer entered the ocean in a succession of grand cascades.! 



VI. Comparison toith the American Submerged Platform. — In 

 my former paper| I described briefly the results arrived at by 

 Professor Spencer and other American geologists regarding 

 the " drowned " plains, escarpments, and river valleys lying 

 outside the North American coast, and I showed that they 

 consist (1) of the " Continental Shelf," stretching out into the 

 Atlantic as far as the 100-fathom line, or thereabouts, where 

 it breaks off" along an escarpment, descending to a depth of 

 450 or 500 fathoms. This escarpment is then succeeded by 

 a second and more extensive terrace, known as " the Blake 

 Plateau," which in turn terminates along a second grand 

 escarpment descending to the abyssal depths of the ocean. 



* The deepest point shown by the soundings is 95 fms., while the 

 bordering level of the sea-bed is 36 fms. 



t Man was not present to view the scene presented by the British 

 Isles at this time ; but we may easily reproduce before our minds its 

 grandeur as visible from the ocean at a distance of a few miles from the 

 coast. In front would rise the lofty terraced cliffs, several thovisand feet 

 in height, and stretching away to the north and south in bold headlands 

 and wide bays till lost to sight in the distance ; while, planted on the 

 nearly level terrace above, would be seen in the far distance the mountain 

 heights of Britain or Ireland robed in a white mantle of snow. 



I On Another Possible Cause of the Glacial Epoch, 1898. (Now preparing 

 for publication.) 



