Prof. Spencer — On the Eastern Margin of JSf. Atlantic. 559 



VI. — Mr. Hudleston " On the Eastern Margin of the North 

 Atlantic Basin." ^ 



By Prof. J. W. Spencer, M.A., Ph.D., B.A.Sc, F.G.S. 



THE delay of several months, in replying to Mr. Hudleston's 

 communication relative to the above-named subject, has been 

 due to absence in the field in a distant region, where I was not in 

 touch with the current literature. 



In the study of suboceanic topography, it is to be regretted that 

 most of the writers use the hydrographic unit of measure — the 

 fathom — where the investigations have for their object the com- 

 parison of the submarine features with those of the land where the 

 unit is in feet. This use of two units dims the comprehension, unless 

 the differential measurements are translated into the same standard. 



On nautical charts, the 10- and 100-fathom contours are generally 

 drawn for navigation purposes, and the latter is the only contour- 

 line that usually approaches the suboceanic continental margins. 

 Hence this line is most commonly spoken of as if identical with, 

 the submerged margin of the continent. This inaccuracy would 

 scarcely obtain were the measurements in feet adopted. The edges 

 of the submarine platforms vary all the way from 300 to 1,200 feet, 

 though the margins of the outer shelves often reach to 3,000 feet 

 or more. 



In the study of the peculiarities of the suboceanic continental 

 margins, contours of 1,000 or 500 fathoms apart are not sufficiently 

 ■close to bring out the features in an intelligent way, although they 

 may be all that are practicable on a map of small scale. By drawing 

 them at 200 or 300 feet apart, I have found that many difficult 

 features became easily explicable. If Mr. Hudleston had taken the 

 fullest data and contoured the slopes from 100 to 1,000 fathoms on 

 his map of the " British Biscayan Atlantic," with isobars of 200 or 

 300 feet, it is possible that he would not have found that the 

 soundings " in certain localities round the Bay of Biscay are curious 

 and perplexing." It is such curious and perplexing features as 

 these, made known to a better degree in more favoured regions, 

 which I have studied in large numbers, and which Professor Hull 

 lias found and more fully studied ofi" the European coast. But 

 Mr. Hudleston, in his really valuable paper, does not seem to have 

 recognized their importance. 



The first striking feature in Mr. Hudleston's paper is his study 

 -of the gradients of the suboceanic continental slopes. He has 

 shown a very large number of cases where the declivities at 

 different points amount to from only 1 in 188 to 1 in 7 or 8, 

 except in the Bay of Biscay, where they may be as much as 1 in 

 4 or 5, and in one case so precipitous as to reach a deepening of 

 one foot in two. His averages are those extending usually over 

 oonsiderable distances of 20 miles or more. The conclusion 

 drawn from them is that the great continental slopes are not 



1 Geol. Mag., Dec. IV, Vol. VI (1899), pp. 97-105 and 145-157. 



