274 PEOF. E. BULL, LL.D., ETC., ON THE SUB-OCEANIC 



abyssal regions of the ocean. Measured from Rockall hx 

 the north (lat. 57° 12' N.) to Cape St. Vincent it has a 

 length of about 1.500 English miles; but along the coast, 

 about 2,000 miles. Such an oceanic slope if elevated into 

 land would have no parallel in Europe ; but, as Professor 

 J. W. Spencer has shown, has its representative in the steep 

 declivity which ranges from Mexico into the United States of 

 America, surmounted by a plateau rising to 8,000 feet 

 above the Gulf of Mexico.* The Great Declivity of the 

 eastern Atlantic is diversified by numerous headlands and 

 bays ; while it is also deeply cleft to its very base by 

 channels intersecting the platform for long distances; being, 

 in fact, canons, or fjords, of the rivers which descended 

 into the ocean Avhen under terrestrial conditions; these will 

 be described further on. 



As far as I have been able to determine from the 

 soundings the Great Declivity descends in one continuous 

 sweep from its upper margin to its base at a depth of (5,000 

 to 9,000 feet below the surface of the ocean. In this respect 

 it differs from its representative on the western borders of 

 the Atlantic, where the descent from the American Con- 

 tinental Shelf into the abyssal regions is accomplished by 

 means of two distinct declivities separated by a broad and 

 well defined terrace, known as the "Blake Plateau."! These 

 differences in the form of the continental slopes on opposite 

 sides of the ocean are illustrated by the following general 

 sections (Figs. 1 and 2). 



The base of the Declivity, which in the case of theVidalBank 

 off the coast of Scotland corresponds with the 1,000-fathom 

 contour, gradually descends in a southerly direction, so that 

 off the Porcupine Bank it appears to touch the 1,500 contour, 

 giving a total descent of 7,bOO feet (1,300 fathoms) from the 

 upper margin to its base. With this general descent the 

 Declivity foUoAvs the coast of the Bay of Biscay and that of 

 Spain and Portugal to Cape St. Vincent, beyond which points 

 I have not as yet had opportunity to follow it ; we cannot 

 doubt, however, that it is represented off" the coast of Africa 

 for some distance southwards. 



From the base of the Great Declivity at a depth varying 

 between 1,000 to 1,500 fathoms — the floor of the abyssal 



* Spencer, "Great Changes of level in Mexico, etc.," Bull. Geol. Soc. 

 Amer., vol. ix (1897) ; Trans. Canadian Inst., vol. v. 



t So-called by Professor L. Agasssiz, Two Cruises of the " Blake.'' 



