August 7, 1890] 



NATURE 



359 



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 looo to 2cxx5 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. 



C. Facts from Regions not Volcanic which are unfavourable fo 

 the idea of a Volcanic Origin. 



1. In the North Pacific, near its centre, the area of 3000 or 

 more fathoms about 35° N. ; the two similar but smaller areas 

 toward its eastern border ; the areas north of the Carolines in 

 the western part of the ocean ; the broad equatorial area about 

 the Phoenix group ; the area in the South Pacific in 170° W., 

 east of Chatham Island, and another just south of Australia, 

 are all so situated that no reason is apparent for referring them 

 to a volcanic origin. Some of the areas are in the coral island 

 latitudes, and the supposed volcanic basis of coral islands makes 

 a volcanic origin possible, but their probable size and position 

 appears to favour the idea of origin through some more funda- 

 mental cause. The area in the South Pacific, east of Chatham 

 Island, is 450 miles distant from the land. The border of 

 southern Australia, abreast of the deep-sea trough, has no 

 known volcano. 



2. In the Atlantic, away from the West Indies. — The 3000- 

 fathom areas of the North and South Atlantic— that is^ the 

 three in the North Atlantic, the two in the South Atlantic, and 

 the two equatorial, one near the coast of Guinea and the other 

 near that of South America — occupy positions that suggest no 

 relation to volcanic conditions. The Cape Verdes, north of the 

 equator, are partly encircled by one of the deep areas, some- 

 what like the eastern end of the Hawaiian group ; but this 

 bathymetric area appears to be too large to owe its origin 

 directly to volcanic work in the group. The coast of Guinea near 

 the 3000-fathom area has nothing volcanic about it, and the 

 opposite coast of South America, near another, is free from 

 volcanoes. 



The only facts in the Atlantic that suggest a volcanic origin 

 are the depression of 2445 fathoms within 40 miles of the west 

 side of the volcanic Cape Verde archipelago, and that of 2060 

 fathoms within 20 miles of Ascension Island ; and a connection 

 is possible. 



3. In and near the West Indies.— The most remarkable of 

 the depths pf the Atlantic area are situated in and near the region 

 of the West Indies, as is well illustrated and discussed by Mr. 

 Alexander Agassiz in his instructive work on the "Three 

 Cruises pf .the ^li^hf." The deepest trough of the ocean, 4561 

 fathoms, occurs within seventy miles of Porto Rico ; and yet 

 this island has no great volcanic mountain, though having 

 basaltic rocks. By the north side of the Bahama belt of coral 

 reefs and inlands, for 600 miles, as Mr. Agassiz well illustrates, 

 the depth becomes 2700 to 3000 fathoms within twenty miles 

 of the coast-line, and at one point 2990 within twelve miles, a 

 pitch-down of i : 3'5 ; and nothing suggests a volcanic cause for 

 the abrupt descent. Cuba and Hayti are not volcanic, and look 

 as if they were an extension of Florida, so that no grounds exist 

 for assuming that the Bahamas rest on volcanic summits. 



One of the strangest of 3000-fathom troughs is that which 

 commences off the south shore of Eastern Cuba, having there 

 a depth of 3000 to 3180 fathoms. It is within 20 miles of this 

 non-volcanic shore, and nearly three times this distance from 

 Jamaica. No sufficient reason appears at present for pronounc- 

 ing its origip volcanic. It is continued in a west-by-south 

 direction to a point beyond the meridian of 85° W., or over 700 

 miles, .making it a very long trough, and the depths vary from 

 2700 tp 3428 fathoms. The depression extends on into the Gulf 

 of Honduras, carrying a depth of 2000 fathoms far toward its 

 head, and in a small indentation of the coast it stops ; for 

 nothing of it appears in the outline of the Pacific coast or the 

 depths off it, and nothing in the range of volcanic mountains on 

 the coast. Against the three deepest parts of the trough there 

 a.re,frst,'the Grand Cayman Reef, 20 miles north of a spot 3428 

 fathoms deep ; second, banks in 13 and 15 fathoms within 15 

 miles of a depth of 2982 fathoms ; and third, Swan Island Reef, 

 15 miles south of a depth of 3010 fathoms ; the first of the three 

 indicating ^ slope to the bottom of i : 5, and the last of l : 4*4. 

 Why these greatest depths in the trough, so abrupt in depression, 

 should be on one side of shoals or emerged coral reefs, it is not 



NO. 1084, VOL. 42] 



easy to explain ; and the more so that the part of the trough 

 south of Cuba has nothing volcanic near by in the adjoining 

 mountain range, and the fact also that the westernmost end of 

 the trough extends on for 175 miles, and there has a depth of 

 3048 fathoms, with 2000 fathoms either side and no coral reefs. 



D. Arrangement of the Deep-sea Troughs in the two halves of the 

 Oceans, pointing to some other than a Volcanic Origin. 



The western half of the Atlantic and Pacific oceans contains 

 much the larger part of the 3000-fathom areas and all the depths 

 over 4000 fathoms. In the North Atlantic the areas of 3000 

 and over in the western half, or off the United States, are very 

 large ; and the bathymetric line of 2500 fathoms extends west- 

 ward nearly to the looo-fathom line. This important feature 

 can be appreciated for both oceans from a look at the map 

 without special explanations. 



As a partial consequence of this arrangement, the Pacific, 

 viewed as a whole, may be said to have a westward slope in its 

 bottom, or from the South American coast toward Japan. This 

 westward slope of the bottom exists even in the area between 

 New Zealand and Australia — the ocean in this area being shallow 

 for a long distance out on the east side and deepening to 2500- 

 2700 fathoms close to that non-volcanic land, New South Wales, 

 in eastern Australia. In the Atlantic, the slope is in the 

 direction of its north-east-south-west axis, either side of the 

 Dolphin Shoal, but especially the western side, rather than from 

 east to west, it commencing in the Scandinavian plateau and 

 ending in the great depths adjoining the West Indies. 



Owing to the system in the Atlantic topography, the Dolphin 

 Shoal — the site of the Atlantis of ancient and modem fable — is 

 really an appendage to the eastern continent, that is to Europe, 

 and is shut off by wide abyssal seas from the lands to the west 

 that have been supposed to need its gravel for rock-making. 



But the view that the west half of an oceanic basin is always 

 the deepest becomes checked by finding in the Indian Ocean 

 that the only areas that are 3000 fathoms deep or over are in 

 the eastern part of the ocean and off the north-west coast of 

 Australia, and near western Java and Sumatra. The greatest 

 depths in its western half or toward Africa, are 2400 to 2600 

 fathoms.^ 



III. Conclusions. 



1. The facts reviewed lead far away from the idea that vol- 

 canic action has been predominant in determining the position of 

 the deep-sea troughs. It has probably occasioned some deep 

 depressions within a score or two of miles of the centre of 

 activity, but beyond this the great depths have probably had 

 some other origin. 



2. It is further evident that the deep-sea troughs are not a 

 result of superficial causes of trough-making. Erosion over the 

 ocean's bottom cannot excavate isolated troughs. The coldest 

 water of the ocean stands in the deep holes or troughs instead of 

 running, as the reader of Agassiz's volume has learned. 



The superficial operation of weighting the earth's crust with 

 sediment, or with coral or other organic-made limestone, and 

 filling the depressions as fast as made, much appealed to in 

 explanations of subsidence, has not produced the troughs ; for 

 filled depressions are not the kind under consideration. More- 

 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 abundant 

 pelagic and sea-border life, and yet the marvellous depths exist. 

 And the depths of the open oceans are no less without explana- 

 tion. 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 manifestly kept 

 themselves at the surface in spite of it.'^ 



3. If superficially acting causes are insuflScient, we are led to 

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



' In the Arctic seas, going north from the Scandinavian plateau, the water 

 deepens north of the latitude of Iceland, between Greenland arid bpitzbergen, 

 to 2000 fathoms, and farther north to 2650 fathoms, in the latitude nearly of 

 Greenwich ; and it is probable that the 2000-fathom area extends over the 

 region of the North Pole. The continents of Europe (with Asia probably) 

 and North America are proved by the shallow .soundings over the adjoining 

 Arctic seas and the islands or emerged land, to extend to about 82}° N., 

 which is about 450 miles from the Pole. 



a The migrations from South America alluded to in note 2 on page 358, 

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

 part of the West India seas afterwardsw^^t^A subsidence in the Quaternary. 

 How far the Bahama and Florida region participated in the subsidence 

 is not kaown. That it did not participate in it has not been proved. 



