554 DEEP-SEA DEPOSITS. 



However worthy of interest the question of life iu the abysses of the 

 sea may be, we shall pass it by here, to study the strictly luiueral sub- 

 stances with which these organized vestiges are usually associated. 



The thing to be noted before all others, in the central parts of the 

 great oceanic basins, is the gradual disappearance of terrigenous 

 dei)osits, which are replaced by volcanic debris. 



This contrast had been perceived as early as 18o(i, when the sound- 

 ings were made in the North Atlantic for laying the telegraphic cable 

 between Ireland and N«nvf(mndland. The idea which had at first pre- 

 sented itself was that the scoriacc^ous silicates brought to the surface 

 might be only the ashes thrown out by the steamers which cross that 

 l)art of the sea in great numbers. But a more careful examination 

 shows well-marked debris of pumice and obsidian. Hence they must 

 be considered as substances thrown up by volcanoes and probably by 

 those not distant from these regions, tliose of Iceland, the Azores, the 

 Antilles, or of Central America. 



At the present time from numerous soundings, made in most varied 

 regions, we learn that volcanic substances are spread generally over 

 the great depths of the ocean. They oftenest form an incoherent, vitre- 

 ous, and spongy material, similar to that designated for a long time by 

 the name of pumice., from the name of the islands where it was found in 

 ancient times. Their spongy texture is due to the gases and vapors 

 which were disengaged from them before their substance was cold or 

 hardened. 



Often also, the substances brought by the dredges from the bottom 

 of the sea are likewise of vitreous nature; but instead of being trachy- 

 tic in composition like the pumice fragments, they are nearer to the 

 basalts. The small fragments or lapilli are of the size of a nut or a 

 pea. 



It is remarkiible that the vitreous texture, comparatively rare in the 

 volcanoes of terra firma, should be so frequent in the volcanic debris 

 which occupy the bottom of the sea, which seems to indicate that cir- 

 cumstances are favorable to its iJioduction in submarine erujitions. 



Mr. Murray (in a communication made to the Royal Society of Edin- 

 burgh iu 1870) was the first to bring to notice the importance and the 

 large i)lace that substances of volcanic origin occupy at great depths. 

 The original character however of the igneous ejections becomes 

 more or less modified under the prolonged action of sea water. While 

 the pumice changes to an earthy and friable matter, the pyroxenic 

 hipiJli produce a substance of brilliant luster similar to that named 

 palafionite by Sartorius and Watterhausen. Some rock materials, such 

 as the pumice fragments, from their porosity float for some time after 

 reaching the surface of the sea before the water gradually penetrates 

 their pores and carries them to the bottom. It is thus that the Chal- 

 lenger often found in its nets fragments of pumice, varying in bulk from 

 the size of a man's head to that of a mustard seed, and usually round 



