MARINE DEPOSITS 209 



times of calcareous and sometimes of siliceous 

 remains, with volcanic mineral particles more 

 or less decomposed, fragments of pumice, 

 grains or nodules of manganese peroxide, 

 earbones of whales, teeth of sharks, zeolitic 

 crystals, etc. Calcareous remains are absent in 

 the red clay from very deep water, but in less 

 depths they may increase in abundance until 

 the deposit merges gradually into globigerina 

 ooze. In like manner siliceous remains may 

 be absent from the red clays of certain regions, 

 but in other localities radiolarian skeletons or 

 diatom frustules may become so abundant 

 that the deposit passes on the one hand into 

 radiolarian ooze and on the other into diatom 

 ooze. Of the inorganic admixtures in the red 

 clays pumice is the most constant and most 

 widely distributed ; it occurs in fragments 

 bigger than a man's head down to the most 

 minute particles, recognisable only under the 

 highest magnifying powers, and in all stages 

 of decomposition — some almost unaltered, 

 others surrounded by zones of alteration, and 

 others so profoundly decomposed as to have 

 lost nearly all trace of their original structure, 

 and often enclosed within a thick coating of 

 manganese peroxide. The crystalline minerals 

 found in pumice, like sanidine, plagioclase, 

 augite, etc., are also characteristic of red 

 clays (as well as of globigerina and pteropod 

 o 



