OCEANIC SOUNDINGS. 19 



trough encircling a shallower central area, whose axis is formed by the Sandwich 

 Islands and the reefs continuing them towards the north-west. The pits named 

 from Wyman (3,300), east of Hawaii, as well as those of Belknap (3,100) and 

 Ammen (3,094), south and west of the same group, belong to this circular depres- 

 sion, which farther west towards Japan takes the name of the Tuscarora, the 

 American ship which here revealed the greatest depth yet recorded in any oceanic 

 basin. This chasm of 4,650 fathoms, sinking nearly as low as the highest moun- 

 tain rises above sea-level, is situated about 240 miles to the east of the southern 

 extremity of the Kurile Archipelago. 



As in the Indian Ocean, the greatest depths woidd thus seem to occur also in 

 the Pacific in the neighbourhood of igneous areas, that is, along the line of active 

 volcanoes which curves round from Japan to the peninsula of Alaska. These 

 chains of burning mountains may thus be said to represent the true coastline of 

 the North Pacific basin. Beyond them the waters are comparatively very 

 shallow, except in one part of the Bering Sea, where depths of 500 and even 

 1,000 fathoms have been recorded. The projecting mainlands of North-east Asia 

 and North-west America rest on a common submarine base, which approaches very 

 near to the surface. In Bering Strait itself the mean depth is little over 20 

 fathoms, and nowhere exceeds 30 fathoms. Between these shallows, here scarcely 

 separating the two continents, and the profound abysses of the North Pacific the 

 transition is very sudden. At some point the soundings have revealed precipitous 

 inclines which would be regarded as steep slopes even in Continental Alpine regions. 



Except in the neighbourhood of California the Eastern Pacific waters have been 

 less carefully surveyed than the Australasian seas. The whole space, some 

 12,000,000 square miles in extent, comprised between the Polynesian archi- 

 pelagoes and the American seaboard from Mexico to (Jhili, was still unsounded 

 before the expedition of the Italian vessel, the Vettor Pisani, in 1885. Now, 

 however, we possess a series of thirteen soundings between the coast of New 

 Grenada and the Sandwich Islands, where 3,140 fathoms was the greatest depth 

 recorded by this expedition. Allowing for the irregularity of the intervals 

 between these soundings, the mean depth of the marine bed in this part of the 

 East Pacific Ocean would appear to be about 2,300 fathoms. Before the Vettor 

 Pisani expedition the velocity of the waves caused by great seaquakes was the 

 only available means for determining the depth of the waters in this section of 

 the oceanic basin. 



The specimens brought to the surface during the various exploring expeditions 

 present on the whole a remarkable uniformity. In the vicinity of the land, and 

 especially about the great fluvial estuaries, the mud and clays of the marine bed 

 are formed by deposits of terrestrial origin mingled with fragments of shells and 

 corals. Farther seaward, in depths ranging from 500 to 1,500 fathoms, the sedi- 

 mentary matter consists of triturated shells and the calcareous remains of animal- 

 culse. The mud dredged in these waters contains from ninety to ninety-five per 

 cent, of carbonate of lime. But according as the depths increase this proportion 

 diminishes, and in abysses of 2,000 to 2,500 fathoms the prevailing formation is 



