198 J. D. Dana — Deep troughs of the Oceanic depression. 



C. I acts from regions not volcanic which are unfavorable to the 

 idea of a volcanic origin. 



1. In the North Pacific, near its center, the area of 3000 or 

 more fathoms about 35° ~N. ; the two similar but smaller areas 

 toward its eastern border ; the areas north of the Carolines in 

 the western part of the ocean ; the broad equatorial area about 

 the Phoenix Group ; the area in the South Pacific in 170° W., 

 east of Chatham Island, and another just south of Australia, 

 are all so situated that no reason is apparent for referring them 

 to a volcanic origin. Some of the areas are in the coral-island 

 latitudes, and the supposed volcanic basis of coral-islands makes 

 a volcanic origin possible, but their probable size and position 

 appears to favor the idea of origin through some more funda- 

 mental cause. The area in the South Pacific, east of Chatham 

 Island, is 450 miles distant from the land. The border of 

 southern Australia, abreast of the deep-sea trough, has no 

 known volcano. 



2. In the Atlantic, away from the West Indies — The 3000- 

 fathom areas of the North and South Atlantic, that is, the 

 three in the North Atlantic, the two in the South Atlantic, and 

 the two equatorial, one near the coast of Guinea and the other 

 near that of South America, occupy positions that suggest no- 

 relation to volcanic conditions. The Cape Verdes, north of the 

 equator, are partly encircled by one of the deep areas, some- 

 what like the eastern end of the Hawaiian group ; but this 

 bathymetric area appears to be too large to owe its origin di- 

 rectly to volcanic work in the group. The coast of Guinea near 

 the 3000-f athom area has nothing volcanic about it, and the op- 

 posite coast of South America, near another, is free from vol- 

 canoes. 



The only facts in the Atlantic that suggest a volcanic origin 

 are the depression of 2445 fathoms within 40 miles of the west 

 side of the volcanic Cape Verde Archipelago, and that of 2060 

 fathoms within 20 miles of Ascension Island ; and a connection 

 is possible. 



3. In and near the West Indies. — The most remarkable of 

 the depths of the Atlantic area are situated in and near the region 

 of the West Indies, as is well illustrated and discussed by Mi* 

 Alexander Agassiz in his instructive work on the " Three 

 Cruises of the Blake." The deepest trough of the ocean, 4561 

 fathoms, occurs within seventy miles of Porto Rico ; and yet 

 this island has no great volcanic mountain, though having ba- 

 saltic rocks. By the north side of the Bahama belt of coral 

 reefs and islands, for 600 miles, as Mr. Agassiz well illustrates, 

 the depth becomes 2700 to 3000 fathoms within twenty miles 

 of the coast-line, and at one point 2990 within twelve miles, a 

 pitch-down of 1 : 3*5 ; and nothing suggests a volcanic cause for 



