J. D. Dana — Deep troughs of the Oceanic depression. 197 



is over 2500 miles. The map is made to suggest its extension 

 still farther eastward ; but among the very few soundings 

 made, none below 8664 fathoms have yet been obtained off 

 the more eastern Aleutians. 



Other similar facts may be found on the map ; and still 

 others may exist which are not now manifest owing to the 

 sinking of oceanic areas and islands. But no cases can be 

 pointed to which are more decisively in favor of volcanic 

 origin. 



B. Facts from the vicinity of volcanic regions apparently not 

 referable to a volcanic origin. 



1. The ocean off the western border of North and South 

 America affords striking examples of the absence of deep 

 troughs from the vicinity of regions eminently volcanic. The 

 South American volcanoes are many and lofty ; and still the 

 ocean adjoining is mostly between 2000 and 2700 fathoms in 

 depth : and just south of Valparaiso, it shallows to 1325 

 fathoms. The only exception yet observed is that of a short 

 trough of 3000 to 3368 fathoms close by the Peruvian shore. 

 It may, however, prove to be a long trough, although certainly 

 stopping short of Valparaiso. The waters, however, of the 

 Pacific border of America deepen abruptly compared with 

 those of the Atlantic border ; and the significance of this fact 

 deserves consideration. 



The facts off Central America are more remarkable than 

 those off the coast to the south. The volcanoes are quite near 

 to the Pacific coast, and still the depths are between 1500 and 

 2500 fathoms. 



The condition is the same off the west coast of N orth Amer- 

 ica. Of the two areas of 3000 and over, nearest to the east 

 coast of the North Pacific, one is 600 miles distant in the lati- 

 tude of San Francisco, and the other is within ten degrees of 

 the equator and twenty degrees of the coast ; both too far 

 away to be a consequence of volcanic action in California, 

 Mexico or Central America. 



In the North Atlantic the European side has its volcanoes 

 and has had them since the Silurian era, and yet the non- 

 volcanic North American side of the ocean has far the larger 

 areas of deep water and much greater mean depth. The 

 Azores or Western Islands, which are all volcanic, have depths 

 around them of only 1000 to 2000 fathoms and no local 

 troughs. Iceland, the land of Hecla, is in still shallower waters, 

 with no evidence of local depressions off its shores. The Cana- 

 ries are volcanic, but no deep trough is near them. 



