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



Art. XXII. — On the Origin, of the deep troughs of the 

 Oceanic depression : Are any of Yolcanic origin f by 

 James D. Dana, with a bathymetric map, Plate VII. 



The consideration of the question with regard to the origin 

 of the ocean's deep troughs requires, as the first step, a gen- 

 eral review of oceanic topography ; for according to recent 

 bathymetric investigations, the deep troughs are part of the 

 system of topography, and its grander part. We need, for 

 this purpose, an accurate map of the depths and heights 

 through all the great area. Such a map will ultimately be 

 made through the combined services of the Hydrographic De- 

 partments of the civilized nations. At the present time the 

 lines of soundings over the oceans, especially over the Pacilic 

 and Indian, are few, and only some general conclusions are at- 

 tainable. It is to be noticed that the system of features of the 

 oceanic area are involved in the more general terrestrial sys- 

 tem ; but since the former comprises nearly three-fourths of 

 the surface of the sphere, it is not a subordinate part in that 

 system. 



With reference to this discussion of the subject I have pre- 

 pared the accompanying bathymetric map. 



I. The bathymetric map, and the general features 

 of the Oceanic depression displayed by it. 



1. The Map. — In the preparation of the bathymetric map 

 I have used the recent charts of the Hydrographic Depart- 

 ments of the United States and Great Britain,* which contain 

 all depths to date, and the lists of new soundings published in 

 German and other geographical Journals. In order that the 

 facts on which the bathymetric lines are based may be before 

 the reader a large part of the depths are given, but in an ab- 

 breviated form, 100 fathoms being made the unit : 25 signify- 

 ing 2500 fathoms or nearly (between 2460 and 2550) ; 2 '3, 

 about 230 fathoms, "4, about 40 fathoms. Only for some deep 

 points is the depth given in full. The addition of a plus sign 

 (+) signifies no bottom reached by the sounding. f 



* I am indebted to the Hydrographic Departments of Great Britain as well as 

 the United States for copies of these charts. 



f On the map the bathymetric lines for 1000, 2000. 3000 and 4000 fathoms, 

 besides being distinguished in the usual way by number of dots, have been made 

 to differ in breadth of line, the deeper being made quite heavy in order to exhibit 

 plainly the positions of the areas without the use of colors. The line for 

 100 fathoms is, as usual, a simple dotted line. As the bathymetric map herewith 



