GEOLOGY OF THE ATLANTIC OCEAN. 41 



GEOLOGY OF THE ATLANTIC OCEAN.* 



Bt Sir WILLIAM DAWSON, 



PRINCIPAL OF M C GILL COLLEGE, MONTREAL. 

 I. 



THE geological history of the Atlantic depression of the earth's 

 crust, and its relation to the continental masses which limit it, 

 may furnish a theme at once generally intelligible and connected with 

 great questions as to the structure and history of the earth, which have 

 excited the attention alike of physicists, geologists, biologists, geogra- 

 phers, and ethnologists. If we imagine an observer contemplating the 

 earth from a convenient distance in space, and scrutinizing its features 

 as it rolls before him, we may suppose him to be struck with the fact 

 that eleven sixteenths of its surface are covered with water, and that 

 the land is so unequally distributed that from one point of view he 

 would see a hemisphere almost exclusively oceanic, while nearly the 

 whole of the dry land is gathered in the opposite hemisphere. He 

 might observe that the great oceanic area of the Pacific and Antarctic 

 Oceans is dotted with islands — like a shallow pool with stones rising 

 above its surface — as if its general depth were small in comparison 

 with its area. He might also notice that a mass or belt of land sur- 

 rounds each pole, and that the northern ring sends off to the southward 

 three vast tongues of land and of mountain-chains, terminating respect- 

 ively in South America, South Africa, and Australia, toward which 

 feebler and insular processes are given off by the Antarctic continental 

 mass. This, as some geographers have observed, gives a rudely three- 

 ribbed aspect to the earth, though two of the three ribs are crowded 

 together and form the Europ-Asian mass or double continent, while 

 the third is isolated in the single Continent of America. He might 

 also observe that the northern girdle is cut across, so that the Atlantic 

 opens by a wide space into the Arctic Sea, while the Pacific is con- 

 tracted toward the north, but confluent with the Antarctic Ocean. The 

 Atlantic is also relatively deeper and less cumbered with islands than 

 the Pacific, which has the higher ridges near its shores, constituting 

 what some visitors to the Pacific coast of America have not inaptly 

 called the " back of the world," while the wider slopes face the nar- 

 rower ocean, into which for this reason the greater part of the drain- 

 age of the land is poured. The Pacific and Atlantic, though both 

 depressions or flattenings of the earth, are, as we shall find, different 

 in age, character, and conditions ; and the Atlantic, though the smaller, 

 is the older, and from the geological point of view, in some respects, 

 the more important of the two. If our imaginary observer had the 

 means of knowing anything of the rock formations of the continents, 



* From the inaugural address of the President of the British Association for the 

 Advancement of Science, delivered at Birmingham, England, September 1, 1886. 



