SECT. 2] TOPOGRAPHY OF THE DEEP-SEA FLOOR 257 



ijattern probably continued through the South Atlantic, Indian and South 

 Pacific Oceans. Individual abyssal hills are identical to the smaller hills rising 

 from the steps of the mid-oceanic ridge and are probably of the same origin. 

 The abyssal hills and the lower step of the Mid-Atlantic Ridge can be dis- 

 tinguished topographically only by the contrast in level. 



Abyssal hills cover a much larger proportion of the Pacific basin than in any 

 other ocean. In certain areas where the relief is in general similar to abyssal 

 hills but of somewhat larger amplitude (i.e. many of the individual peaks 

 exceed 500 fm in height), the term "seamount province" has been used. A 

 good example of such an area is the Baja California seamount province (Menard, 

 1955), where 220 seamounts have been found in an area of about 1,000,000 

 square miles, which is hilly between seamounts. Echograms have been obtained 

 along api^roximately 50,000 miles of track in this province and no extensive 

 smooth bottom has been found except for a very limited area at the base of 

 Patton Escarpment. Detailed surveys off the United States show groups of 

 ridges averaging 20 x 100 miles and a few hundred fathoms of relief. These 

 low, linear mountains trend north-south. In contrast, about 50 of the higher 

 seamounts have been surveyed and all appear to be volcanoes although many 

 are elongate like the Hawaiian volcanoes. Detailed surveys around taut buoys 

 or with precision positioning have been made of abyssal hills in several places 

 in the Pacific. Although they are linear off California, the hills in the equatorial 

 Pacific are low, irregular domes and are probably formed by laccolithic in- 

 trusions. Statistical analysis of echograms in abyssal-hill areas also indicates 

 that low domes are extremely common. 



d. Abyssal gaps 



If two adjacent, but distinct, abyssal plains have no through passage at or 

 below the level of the higher plain, they are said to be separated by a siU, a 

 ridge or a rise, depending on the dimensions of the features involved. However, 

 several plains are connected by constricted passages. An abyssal gap is a 

 passage connecting two abyssal plains lying at different levels in the vicinity 

 of the gaj). The sea floor slopes down continuously through the gap from the 

 higher to the lower abyssal plain at a gradient considerably greater than that 

 of either adjacent abyssal plain. Prominent abyssal gaps are known in the 

 western Atlantic (Vema Gap) and in the eastern North Atlantic (Theta Gap) 

 and an abyssal gap connects Colombia and the Venezuela abyssal plains of the 

 Caribbean. Investigations of both Vema Gap and Theta Gap have shown that 

 small mid-ocean canyons (deep-sea channels) connect the upper plain with 

 the lower plain. Sediments collected in abyssal gaps have suggested that 

 turbidity currents have transported material through the gaps. 



e. Mid- ocean canyons 



Cascadia Deep-Sea Channel has been traced as a continuous feature from the 

 submarine canyon at the mouth of the Columbia River to a point 600 miles to 

 the west. A few sounding lines indicate that its total length is about 1000 miles. 



