36 PRINCIPLES OF PALAEONTOLOGY. 



the Foraminifera, 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 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 geologi- 

 cal formations are composed mainly 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 clay 

 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, however, render it 

 not improbable that some of the great argillaceous accumulations 

 of past geological periods may be really organic. This dis- 

 tinguished 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 shells of 

 Foraminifera, gave place at still greater depths to a red ooze con- 

 sisting of impalpable clayey mud, colored 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 Thomson came 

 to the conclusion that it was probably formed by the action of the 

 sea-water upon the shells of Foraminifera. These shells, though 

 mainly consisting of lime, also contain a certain proportion of 

 alumina, the former being soluble in the carbonic acid dissolved 

 in the sea-water, whilst the latter is insoluble. There would 

 further appear to be grounds for believing that the solvent power 

 of the sea-water over lime is considerable increased at great 

 depths. If, therefore, we suppose the shells of Foraminifera 

 to be in course of deposition over the floor of the Pacific, at 

 certain depths they would remain unchanged, and would ac- 

 cumulate to form a calcareous ooze; but at greater depths they 

 would be acted upon by the water, their lime would be dis- 

 solved out, their form would disappear, and we should simply have 

 left the small amount of alumina which they previously contained. 



