THE FLOOR OF THE OCEAN BOSWELL 283 



depth cannot be explained by detrital accumulation. Moreover, as 

 stated above, red clays (which are the richest of the deposits) are 

 believed to accumulate exceedingly slowly from decomposition of 

 volcanic material, diluted perhaps by residues from skeletons of organ- 

 isms and rare wind-borne detritus. Piggot suggests that although 

 some uranium may be brought down by the settling of skeletal remains 

 and volcanic dust, the greater proportion comes out as a result of 

 oxidation at depth, for the oxygen concentration in sea water increases, 

 according to the Carnegie's records, with depth below 1,000 fathoms. 

 On the other hand, in shallow water or near to continents sufficient 

 organic matter is present to maintain the reducing conditions, which 

 tend to keep the uranium in solution. The environment at the 

 bottom of the ocean being of an oxidizing rather than reducing nature, 

 organic material disappears, and the water must be almost at satura- 

 tion with respect to oxides of uranium. Thus there is a tendency for 

 them to separate out like the oxides of iron and manganese. 



The data obtained by the Carnegie regarding this increase in oxy- 

 gen concentration throws light on various other phenomena and 

 even on the color and composition of the deepest deposit — red clay 

 itself, but they raise a difficulty when we try to explain the fresh and 

 unoxidized condition of many of the minerals in deep-sea deposits. 

 A considerable number of these minerals, as well as the fragments 

 of volcanic glass associated with them, are of a type that is far from 

 resistant to the effects of weathering, such as oxidation and hydra- 

 tion, on land. Reference is made below to the volcanic origin of 

 most of such minerals. Well-known examples like olivine, augite, 

 hypersthene, and biotite mica, are found in deep-sea sediments in a 

 fresh unaltered state, often with crystal faces still sharply defined. 

 Submarine "weathering," to use a contradictory term, cannot there- 

 fore proceed on the same lines as the corresponding process on the 

 continents. If the conditions in deep waters are oxidizing, we may 

 ask what it is that inhibits the process analogous to weathering (the 

 "halmyrolysis" of some authors). We can only suggest possible 

 explanations of this "stabilized" condition of affairs; for example, 

 that (a) the waters are charged to saturation with carbon dioxide, 

 (b) they are kept at an approximately low and relatively constant 

 temperature, (c) the pressure is great, often exceeding 500 atmos- 

 pheres, or even (d) light is absent. 



From the foregoing, it will have become evident that there are 

 three possible sources for the mineral material of deep-sea sediments 

 deposited beyond the reach of river-borne detritus, namely, (a) the 

 products of volcanic eruptions, either submarine or wind-borne from 

 terrestrial outbursts, (b) nonvolcanic dust carried by wind from dis- 

 tant lands, and (c) detritus dropped from melting icebergs at extreme 



