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



available were " sufficient to ensure the accuracy of such deep 

 casts."* 



2. The Feature-lines of the oceanic and oordering lands. — 

 The courses of island-ranges and coast lines have a bearing on 

 the question relating to the courses of the deep-sea troughs, 

 and, therefore, by way of introduction, they are here briefly re- 

 viewed, f The system of trends in feature-lines takes new sig- 

 nificance from a bathymetric map, for the courses are no 

 longer mere trends of islands or emerged mountain peaks ; 

 they are the trends of the great mountain ranges themselves ; 

 and, in the Pacific, these mountain courses are those of half a 

 hemisphere. Some of the deductions from such a map are 

 briefly as follows : 



(1). Over the Pacific area there are no prominent north-and- 

 south, or meridional, courses in its ranges, and none over the 

 Atlantic, except the axial range of relatively shallow water in 

 the South Atlantic. And, to this statement it may perti- 

 nently be added that there are none in the great ranges of 

 Asia and Europe, excepting the Urals ; none in North Amer- 

 ica ; none in South America, excepting a part of those oil its 

 west side. 



(2). The ranges in the Pacific ocean have a mean trend of 

 not far from northwest-by-west, which is the course very 

 nearly of the longer diameter of the ocean. One tranverse 

 range crosses the middle South Pacific — the New Zealand — 

 commencing to the south in New Zealand and the islands 

 south of it, with the course N. 35° E., and continuing through 

 the Kermadec Islands and the Tonga group, the latter trend- 

 ing about N. 22° E., and this is the nearest to north and south 

 in the ocean, except toward its western border. 



(3). The oceanic ranges are rarely straight, but instead, 

 change gradually in trend through a large curve or a series of 

 curves. For example, the chain of the central Pacific becomes 

 to the westward, north-northwest ; and the Aleutian range and 

 others off the Asiatic coast, make a series of consecutive 

 curves. Curves are the rule rather than the exception. More- 

 over, the intersections of crossing ranges, curved or not, are 

 in general nearly rectangular. 



(i). Approximate parallelisms exist between the distant 

 ranges or feature-lines ; as (1) between the trend of the New 



* The communication received from the Admiralty Office adds that il Some of 

 Ross's soundings up to 2660 fathoms have been proved correct, and hence the 

 sounding in 68° S., referred to, has been retained on our charts until disproved." 

 "Another sounding obtained by Ross in the Atlantic has had stroDg doubts 

 thrown upon it by a sounding of 3000 fathoms obtained not very far from its 

 position." See the accompanying map. near latitude 14° S. 



f This subject of the system in the earth's feature-lines is presented at length, 

 with a map, in my Expedition Geological report, pp. 11-23 and 414-424; and 

 also more briefly in this Journal, II, ii, 381, 1846. 



