CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 25 



of organic ,origin. It has been shown, namely by obser- 

 vations carried out in our present seas that the shells of 

 Foraminifera are liable to become completely infiltrated by 

 silicates (such as " glauconite," or silicate of iron and 

 potash). Should the actual calcareous shell become dis- 

 solved away subsequent to this infiltration as is also 

 liable to occur then, in place of the shells of the For- 

 aminifera, we get a corresponding number of green sandy 

 grains of glauconite, each grain being the cast of a single 

 shell. It has thus been shown by Dr Carpenter that 

 the green sand found covering the sea-bottom in certain 

 localities (as found by the Challenger expedition along 

 the line of the Agulhas current) is really organic, and is 

 composed of casts of the shells of Foraminifera. Long 

 before these observations had been made, it had been 

 shown by Professor Ehrenberg that the green sands of 

 various geological formations are often composed in part of 

 the internal casts of the shells of Foraminifera; and we 

 have thus another and a very interesting example how rock- 

 deposits of considerable extent and of geological importance 

 can be built up by the operation of the minutest living 

 beings. 



As regards argillaceous deposits, containing alumina or 

 day as their essential ingredient, it cannot be said that any 

 of these have been actually shown to be of organic origin. 

 A recent observation by Sir Wyville Thomson would, how- 

 ever, render it not improbable that some of the great argil- 

 laceous accumulations of past geological periods may be 

 really organic. This distinguished observer, during the 

 cruise of the Challenger, showed that the calcareous ooze 

 which has been already spoken of as covering large areas 

 of the floor of the Atlantic and Pacific at great depths, 

 and which consists almost wholly of the sheila of Foraminif- 

 era, gave place at still greater depths to a red ooze consist- 

 ing of impalpable clayey mud, coloured by oxide of iron, 

 and devoid of traces of organic bodies. As the existence of 

 this widely diffused red ooze, in mid -ocean, and at such 

 great depths, cannot be explained on the supposition that it is 

 a sediment brought down into the sea by rivers, Sir Wyville 



