560 Professor J. W. Spencer — Reply to Mr. ITudleston — 



vertical or nearly vertical precipices, which neither Professor Hull 

 nor I ever stated, and, therefore, much of Mr. Hudleston's labour 

 for the apparent purpose of refutation is lost. But on analyzing 

 his method, it is found superficial only, as it does not give us- 

 a true idea of the variable forms and pitch of the continental slope. 

 In other regions, notably the Floridian channel, leading to the Gulf 

 of Mexico, the analyses of the soundings show that the mean slope- 

 in no way conveys an impression of the actual form of the deepening 

 of the channel. The floor of the submerged valley is in reality 

 made up of a series of very gentle gradients, or almost horizontal 

 steps, separated by precipitous declivities. The margin of the- 

 Mexican plateau — back of Vera Cruz — has an elevation of somewhat 

 more than 8,000 feet above the sea, or 6,500 feet above the inner 

 margin of the coastal plain. Following up one indentation, ther 

 average declivity is only 1 in 30, while at another point it is 

 1 in 10. But these averages do not suggest the declivity of the 

 great escarpment, for the slope is made up of a series of steep 

 gradients, often 1 in 3 or 4, or perhaps in places 1 in 2, separated 

 by zones of comparatively small pitch. The soundings along the 

 Atlantic margin, which Mr. Hudleston has studied, may not have 

 been close enough to reveal the steps, although he has shown that 

 in the Bay of Biscay the declivities are precipitous. It appears to 

 me that his citations of the mean declivity of the continental slope^ 

 scarcely become a criticism of the characteristics of the great 

 drowned valleys, as pointed out by Professor Hull and myself. 



A second feature made by his studies of the suboceanic continental 

 slope is that it is continuous over great distances (which is perfectly 

 correct), and that the Norwegian basin is an extension of that of the 

 North Atlantic ; or, as expressed by him, " it is obvious from an 

 inspection of the accompanying map that the eastern part of the 

 North Atlantic, the Norwegian Atlantic, and a portion of the North 

 Polar ocean, all belong to one and the same great geosynclinal 

 depression, which has been partly interrupted by volcanic extra- 

 vasation, more especially in the region between Shetland and 

 south-east Greenland." This view cannot pass unchallenged. The 

 (" eruptive ") barrier extends practically from Europe to Greenland ;. 

 and between points where the depth of even the valley - like 

 extensions of the basins is 12,000 feet the breadth of the barrier is 

 600 miles or more, and it rises to within 2,000 feet of the surface 

 of the ocean. Such a barriei-, rising 10,000 feet out of 12,000' 

 feet, is certainly too stupendous to be considered as only a partial 

 interruption in one oceanic basin. As a matter of fact there are two- 

 basins (and the more northern is apparently continuous with that 

 of the great Polar Sea), just as the Caribbean and Mediterranean 

 basins are distinct from that of the Atlantic. If we could regard 

 this Icelandic plateau in the manner that Mr. Hudleston does, as 

 a volcanic extravasation, interrupting the continuity of the Atlantic 

 basin, it might obviate the necessity of explaining the valley-likfr 

 features of the ridge. He further says : " to the westward, beyond 

 the centre of Lightning channel (' which is that eastward of Faro& 



