THE DEEP SEA A\D ITS CONTEXTS. 327 



form into a deep sea-bed, or of a deep sea-bed into a continental 

 platform, has received a most unexpected and explicit confirma 

 tion from the study of the Jefrisifs at present being formed on the 

 Oceanic sea bed, of which a sample was brought up in every 

 sounding taken by the Challenger, whilst larger collections of 

 them were made by tiie trawl and the dredge, l- or such deposits 

 as are obviously formed by the disintegration of ordinary land- 

 masses were, as a rule, only found in the comparatively shallow 

 waters in the near neighbourhood of those masses, the almost 

 universal absence of the ordinary siliceous sand of our shores being 

 a most noteworthy fact. Indeed, the exception served to prove 

 the rule ; for it was only when the Challenger s course lay parallel 

 to the coast of Africa, some two or three hundred miles to the 

 westward of it, that the soundings g.ive evidence of its presence; 

 and that this sand had been blown over the sea-surface from the 

 Sahara was indicated by its deposit as a fine dust on the ship s 

 deck : but deposits of volcanic origin were met with in unexpected 

 abundance, the most common being a red clay, first found on the 

 deepest areas of the Atlantic, the source of which was for some 

 lime a question of great perplexity to the scientific start&quot; of the 

 Challenger, from its presenting itself at such a distance irom any 

 land that it could not be supposed to have been brought down 

 (as the clay deposits of shore-waters are) by continental rivers. 

 The clue to the solution of the dilliculty was furnished by the 

 unexpected capture, in the &quot; tow-net, of a considerable number 

 of floating masses of pumice-stone, whilst the trawl frequently 

 brought up bushels of such, varying in size from that of a pea to 

 that of a football. Now pumice is formed of ordinary lava which 

 has been raised (like dough) into a spongy condition by the 

 liberation of gases in its substances, and -it contains a considerable 

 proportion of feldspar, which affords the material of clay ; and as 

 the &amp;lt;iay deposits were found to contain fragments of pumice in 

 various st.igcs of disintegration, the probability of their volcanic 

 origin seems so strong as to justify its full acceptance. Mr. 

 Murray thinks it likely that not only all the pieces of pumice 

 which float on the surface, but those spread over the sea-bottom, 

 have been ejected from /WW-volcanoes ; some of them, perhaps, 

 having fallen irAo the sea in the first instance, but the greater 



