J. D. Dana — Deep troughs of the Oceanic depression. 199 



the abrupt descent. Cuba and Hayti are not volcanic, and look 

 as if they were an extension of Florida, so that no grounds exist 

 for assuming that the Bahamas rest on volcanic summits. 



One of the strangest of 3000 fathom troughs is that which 

 commences off the south shore of Eastern Cuba, having there 

 a depth of 3000 to 3180 fathoms. It is within 20 miles of 

 this non-volcanic shore, and nearly three times this distance 

 from Jamaica. No sufficient reason appears at present for pro- 

 nouncing its origin volcanic. It is continued in a west-by-south 

 direction to a point beyond the meridian of 85° W. or over 700 

 miles, making it a very long trough, and the depths vary from 

 2700 to 3428 fathoms. The depression extends on into the 

 Gulf of Honduras, carrying a depth of 2000 fathoms far to- 

 ward its head, and in a small indentation of the coast it stops ; 

 for nothing of it appears in the outline of the Pacific coast or 

 the depths off it, and nothing in the range of volcanic moun- 

 tains on the coast. Against the three deepest parts of the 

 trough there are, first, the Grand Cayman reef, 20 miles north 

 of a spot 3428 fathoms deep ; second, banks in 13 and 15 

 fathoms within 15 miles of a depth of 2982 fathoms ; and third, 

 Swan Island reef, 15 miles south of a dej3th of 3010 fathoms ; 

 the first of the three indicating a slope to the bottom of 1:5, 

 and the last of 1 :4*4. Why these greatest depths in the trough, 

 so abrupt in depression, should be on one side of shoals or 

 emerged coral reefs, it is not easy to explain ; and the more so 

 that the part of the trough south of Cuba has nothing volcanic 

 near by in the adjoining mountain range, and the fact also that 

 the westernmost end of the trough extends on for 175 miles, 

 and there has a depth of 3048 fathoms, with 2000 fathoms 

 either side and no coral reefs. 



D. Arrangement of the deep sea troughs in the two halves of the 

 oceans, pointing to some other than a volcanic origin. 



The western half of the Atlantic and Pacific oceans contains 

 much the larger part of the 3000-fathom areas and all the 

 depths over 4000 fathoms. In the JSTorth Atlantic the areas of 

 3000 and over in the western half, or off the United States, 

 are very large ; and the bathymetric line of 2500 fathoms ex- 

 tends westward nearly to the 1000-fathom line. This im- 

 portant feature can be appreciated for both oceans from a look 

 at the map without special explanations. 



As a partial consequence of this arrangement, the Pacific, 



viewed as a whole, may be said to have a westward slope in its 



bottom, or from the South American coast toward Japan. This 



westward slope of the bottom exists even in the area between 



Am. Joue. Sci. — Third Series, Vol. XXXVII, No. 219.— March, 1889. 

 13 



