148 W. H. Rudleston — E. Margin of N. Atlantic Basin. 



platform, at a lower level for the most part than the usual 

 continental shelf. This has a very irregular surface, with depths 

 ranging from 130 to 240 fathoms, and it has its termination, 

 seawards in the dix'ection of the Porcupine bank. This irregular 

 platform causes a westwai'd bulge of the Continental submergence, 

 w^hich here presents an edge which is also very irregular and 

 unequal in its depths. In places the suboceanic slope is pretty 

 steep, especially where it faces the north. Thus we find a drop 

 from 135 to 1,350 fathoms in a distance of 23 miles, giving an 

 inclination of 1 in 17, or 3^°. I cannot find anything steeper than 

 this in connection with the Porcupine bank, although it has been 

 stated that the slopes are precipitous. 



Before going south of the Porcupine bank, it may be convenient 

 to consider for a moment the possible geological history of the 

 Atlantic shallows, as we may term that part of the ocean which 

 lies betwixt the British Isles and the south-east coast of Greenland. 

 That some sort of a bridge across the Atlantic may have existed 

 during portions of Tertiary time in these quarters seems not an 

 incredible supposition. Both botanists and zoologists require it. 

 We have already seen that an elevation of 2,000 feet would suffice 

 to effect a land communication between the British Isles and East 

 Greenland at the present day. In a volcanic region like this we 

 may readily concede such a communication by means of sporadic 

 islands, involving local uplifts to the amount of 2,000 or even 

 3,000 feet, without postulating such a general uplift as would make, 

 for instance, the coastline of the North of Scotland coincide with 

 the 500-fathom line. No doubt such a state of things is regarded as 

 probable by eminent British geologists, who differ chiefly as to the 

 period when this occurred. It is impossible on the present occasion 

 to do more than allude to such views, and I am by no means 

 prepared to deny that something of the kind may have taken place 

 in Middle Tertiary time, though I fail to perceive the absolute 

 necessity for it. With respect to the general effect of the partial 

 bridging of the Atlantic upon the theory of the permanence of 

 ocean basins, bearing in mind the volcanic nature of the region in 

 question, we may regard the Atlantic shallows as an oscillating 

 area which lies athwart the general trend of the great meridional 

 depression. Under certain circumstances this oscillating area may 

 break the continuity of the great Atlantic depression, and even 

 bridge it for a time. 



It would be interesting at this stage of the inquiry to ascertain 

 how far the views of Professor Spencer, as stated in a recent 

 communication to the Geological Magazine, are really applicable 

 to this side of the Atlantic ; and whether a study of the Admiralty 

 charts affords any confirmation of his statements or his theories. 

 It is not necessary to refer to the other side of the Atlantic beyond 

 remarking that Professor Spencer, having long studied the region of 

 the Caribbean Sea — perhaps the most unstable on the surface of 

 the earth — appears to have conceived exaggerated notions as to 

 ' epeirogenic ' uplifts on both sides of the Atlantic. An elevation 



