MID-ATLANTIC RIDGE 



217 



cal chart of the oceans, referred to above, and that part of the chart show- 

 ing the ridge is reproduced here in plate 4. 



The persistence with which this feature maintains a medial position 

 in the ocean bed for nearly 9,000 miles (following its great curves) is 

 very striking, and the position which it takes in passing between South 

 America and Africa is still more remarkable. The ridge is a submerged 

 mountain range of a different type and origin from any other on the 

 earth. It is apparently a sort of horst ridge — a residual ridge along a 

 line of parting or rifting — the earth-crust having moved away from it on 

 both sides. On the chart it is indicated to be mostly less than 2,000 

 fathoms (12,000 feet) beneath the sea, with some parts less than 9,000 

 feet and a few volcanic islands which rise 3,000 or 4,000 feet above the 

 sea. Its general height above the surrounding ocean floor is between 

 3,000 and 6,000 feet, with a few island peaks rising 18,000 or 20,000 feet. 

 Between South America and Africa the ridge runs east-southeast through 

 40 degrees of longitude (nearly 3,000 miles), and in this part is parallel 

 with the adjacent continental border of South America. 



The great westward bulge of Africa north of the equator appears to fit 

 very closely into the westward bend of the mid- Atlantic ridge, suggesting 

 that Africa has drifted eastward from that position. All authorities 

 seem to agree, however, that Africa did not participate in the Tertiary 

 folding, but remained stationary. Hence, if its western margin ever 

 rested on the mid-Atlantic ridge it can hardly have been at a later time 

 than the Carboniferous, for no important or extensive crustal movements 

 appear to have affected Africa since that time. Too little is known of 

 the geology of Africa, however, for settled conclusions now, but the nar- 

 row belt of peripheral folds on the southeastern border and also the high 

 plateau of eastern Africa seem to be normal products of a crustal move- 

 ment from the west-northwest. 



At a first glance, the mid- Atlantic ridge appears to favor the torsion 

 hypothesis of Prinz,^ in which the southern hemisphere is supposed to 

 have been rotated to the east relatively to the northern. The ridge cer- 

 tainly seems to suggest such a movement more definitely and precisely 

 than any other feature yet described, but its form would seem to indicate 

 til at the yielding to the torsionary force was confined to a narrow zone 

 near the equator, and tliat in that zone it had caused a displacement of 

 nearly 3,000 miles. If the ridge ran originally straight north and south, 

 and has been offset by torsion to give it its present form, then a section 



8 Dana's Manual of Geology, 4th ed., pp. 395-396. Also Bulletin of the Geological 

 Society of America, vol. 11, 1899, pp. 93-94. 



