J. D. Dana — Deep troughs of the Oceanic depression. 201 



over, the areas are out of the reach of continental sediments 

 and too large and deep to come within the range of possibilities 

 of organic sedimentation or accumulation. The existence of 

 the troughs is sufficient proof of this. The deep troughs of 

 the West Indian and adjoining seas are in a region of abun- 

 dant pelagic and sea-border life, and yet the marvelous depths 

 exist. And the depths of the open oceans are no less without 

 explanation. Those close by the Bahamas extending down to 

 16,000 and 18,000 feet, are evidence of great subsidence from 

 some cause; and the coral reefs for some reason have man- 

 ifestly kept themselves at the surface in spite of it.* 



3. If superficially acting causes are insufficient, we are Jed 

 to look deeper, to the sources of the earth's energies, or its in- 

 terior agencies of development, to which the comprehensive 

 system in its structure and physiognomy points. Whatever 

 there is of system in the greater feature lines, whether marked 

 in troughs or in mountain chains, or island ranges, must come 

 primarily from systematic work within. The work may have 

 been manifested in long lines of flexures or fractures as 

 steps in the process, but the conditions which gave directions 

 to the lines left them subject to local causes of variation, and 

 between the two agencies, the resulting physiognomy has 

 been evolved. 



We have from the Pacific area one observation of a volcanic 

 nature bearing on the comprehensiveness of the system of feat- 

 ure-lines in the oceans, and although I have already referred 

 to it, I here reproduce the facts for use in this place. 



If the ranges of volcanic islands were, in their origin, lines 

 of fissures as a result of comprehensive movements, the lines 

 should continue to be the courses of planes of weakness in the 

 earth's crust. The New Zealand line, including the Kermadec 

 Islands and the Tongan group, has been pointed to as one of 

 these lines, and one of great prominence, since it is the chief 

 northeastward range of the broad Pacific,' and nearly axial to 

 the ocean. The series of volcanoes along the axis of New 

 Zealand is in the same line. It was noticed, at the Tarawera 

 eruption of 1883, that four or five days after the outbreak, 

 and three after it had subsided, White Island, in the Bay of 

 Plenty, at the north end of the New Zealand series, became 

 unusually active ; and two ?nonths later there was a violent 

 eruption in the Tonga group, on the Island of Niuafou. The 

 close relation in time of the latter to the New Zealand erup- 



* The migrations from South America alluded to in a note to page 105, proving 

 an elevation of 2000 feet to make it possible, prove also that a large part of the 

 West India seas afterward suffered subsidence in the Quaternary. How far the 

 Bahama and Florida region participated in the subsidence is not known. That it 

 did not participate in it has not been proved. 



