254 THE ATLANTIC. [chap. v. 



SO abundant are such fragments in some places that the fine 

 amorphous matter, which may be regarded as the ultimate and 

 universal basis of the deposit, appears to be present only in 

 small proportion. 



Mr. Murray has studied very carefully the distribution of vol- 

 canic debris over the floor of the ocean. He finds that recog- 

 nizable pieces of pumice, varying from the size of a pea to that 

 of a foot-ball, have been dredged at eighty stations, distributed 

 all along our route ; and he finds them in greater abundance 

 in the neighborhood of volcanic centres, such as the Agores 

 and the Philippines, than elsewhere. In deposits far from 

 land they were most numerous in the pure deep-sea clays ; mi- 

 nute particles of feldspar, having the a23j)earance of disinte- 

 grated pumice, were detected in all such ocean deposits. Many 

 of the large pieces were much decomposed, while some were 

 only slightly altei'ed ; some were coated with manganese and 

 iron, and many appeared as a mere nucleus, round which the 

 manganese and iron had aggregated. They varied greatly in 

 structure, being highly vesicular, or fibrous and compact, and 

 in color from white through gray or green to black. There 

 seemed to be every gradation from the feldspathic to the ex- 

 treme pyroxenic varieties. 



Mr. Murray believes that all the pieces of pumice which we 

 find at the bottom of the sea have been formed by subaiirial 

 volcanic action. Some of them may have fallen uj)on the sea ; 

 but the great majority seem to have fallen on land, and been 

 subsequently washed and floated out to sea by rains and rivers. 

 After floating about for a longer or shorter time, they have 

 become water-logged and have sunk to the bottom. Both in 

 the North Atlantic and in the Pacific small pieces of pumice 

 were several times taken on the surface of the ocean by means 

 of the tow-net. Over the surface of some of these, serpulte and 

 algse were growing, and crystals of sanidine projected, or were 

 imbedded in the feldspar. During om- visit to Ascension, there 



