318 HEEZEN AND LAUGHTON [CHAP. 14 



(Fig. 2). It is connected on the west by a narrow gap to the Hatteras Plain. 

 This gap, discovered by Vema and known as the Vema Gap, has much higher 

 gradients than the abyssal plains lying both to the northeast and southwest. 

 From 62° to 64''W the northern and southern boundaries of the Nares Abyssal 

 Plain are abrupt and only three or four small abyssal hills have been observed 

 in the plain. East of 64°W the gradient in the plain exceeds 1 : 2500 and the 

 frequency of hills increases until, at 60°W, the plain consists of a series of 

 lingers which extend into the abj^ssal hills. The Nares Abyssal Plain is the 

 deepest of the broad abyssal ])lains of the North Atlantic Ocean Basin floor. It 

 is also the farthest from land. If the material forming the Nares Abyssal Plain 

 was transported along the sea floor, it would have had to flow through Vema 

 Gap, since the outer ridge and marginal trench to the south and the outer 

 ridge and marginal basin to the west prevent bottom-seeking currents from 

 either the south or west from reaching the Nares Abyssal Plain. The idea that 

 the sediment was transported on the bottom through Vema Gap is supported 

 by the fact that the abyssal plain slopes eastward through the gap. 



d. Demerara Abyssal Plain 



A 1000-mile-long abyssal plain lies east of the Lesser Antilles and the 

 Venezuelan and Guianan coasts of South America. The plain varies from 100 

 to 150 mi in width and includes an area of 130,000 mi'^. The Ceara Abyssal Plain 

 lies along the same trend southeast of the Demerara Abyssal Plain. The two 

 plains are separated by the 100-mile-wide Ceara Rise. 



e. Ceara Abyssal Plain 



The Ceara Abyssal Plain, which lies parallel to the coast of the Ceara Pro- 

 vince, Brazil, covers an area of 45,000 mi'. It does not seem to connect either 

 with the Demerara Abyssal Plain to the northwest or with the Pernambuco 

 Abvssal Plain to the southeast. 



B. Eastern North Atlantic 



The abyssal plains of the eastern North Atlantic differ from those of the 

 western side in being smaller and more numerous. This is partly due to 

 the greater complexity of large features in the eastern ocean basin, such as the 

 Horseshoe group of seamounts west of Gibraltar, and partly to the lower 

 supply of sediments from the adjacent continent (Fig. 6). 



a. Porcupine Abyssal Plain 



This plain occupies the deepest part of the shaUow trough between the 

 continental shelf SW of Ireland and the east flank of the Mid-Atlantic Ridge. 

 It covers 10,000 mi^ and is elongated in a NNW-SSE direction. Its slope is un- 

 certain owing to lack of accurate soundings, but it appears to be connected to 

 the western arm of the Biscav Abvssal Plain. 



