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



secondly, those from similar regions that are not favorable to 

 such an origin ; thirdly, facts from other regions bearing on 

 the question. 



A. Facts apparently favoring a volcanic origin. 



1. The Pacific soundings have made known the existence of 

 two deep-sea depressions, if not a continuous trough, within 



forty miles of the Hawaiian Islands ; one situated to the 

 northeast of Oahu, or, north of Molokai, with a depth of 3028 

 fathoms, or 18,069 feet, and the other east of the east point of 

 Hawaii, 2875 fathoms, or within 750 feet of 18,000 feet. 

 Again, 450 miles northeast of Oahu, there is a trough in the 

 ocean's bottom, over 800 miles long, which runs nearly paral- 

 lel with the group and has a depth of 3000 to 3540 fathoms ; 

 and, as far south, another similar trough of probably greater 

 length has afforded soundings of 3000 to 3100 fathoms. The 

 depths about the more western part of the Hawaiian chain of 

 islands have not yet been ascertained, and hence the limits of 

 the deep areas are not known. Such depths, so close to a line 

 of great volcanic mountains, the loftiest of the mountains not 

 yet extinct, appear as if they might have resulted from a sub- 

 sidence consequent on the volcanic action. 



The subsidence might have taken place (1) either from 

 underminings : — which the amount of matter thrown out and 

 now constituting the mountain chain, with its peaks of 20,000 

 to 30,000 feet above the sea-bottom, shows may be large; or 

 (2) from the gravitational pressure in the earth's crust, about a 

 volcanic region which speculation makes a source of the ascen- 

 sive force and of the upward rising of the lavas, — the sub- 

 siding crust following down the liquid surface beneath. In 

 either case the mass of ejected material might be a measure 

 more or less perfectly of the maximum amount of subsidence. 



2. In the western part of the North Pacific, at the south 

 end of the volcanic group of the Ladrones off the largest 

 island of the group, Guam, the Challenger found a depth of 

 4475 fathoms, one of the two deepest spots yet known in the 

 Pacific. The situation with reference to the group is like that 

 off the east end of the Hawaiian group. 



3. East of Japan and the Kuriles, a region of ranges of vol- 

 canoes, there is the longest and deepest trough of the ocean, 

 the length 1800 miles, the depths 4000 to 4650 fathoms ; and 

 farther northeast, south of one of the Aleutian islands, a depth 

 of 4000 fathoms occurs again ; and depths of 3100 to 3664 

 fathoms also still farther east. It is probable that the 4000- 

 line trough continues from the Kurile to this deep spot off the 

 Aleutian volcanic range ; and if so, the length of the trough 



