THE FOSSILIFEROUS ROCKS. 35 



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 maybe really organic. 

 This distinguished observer, during the cruise of the Chal- 

 lenger, 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 consisting 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 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 considerably 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. In process of time this alumina would accumulate 

 to form a bed of clay; and as this clay had been directly 

 derived from the decomposition of the shells of animals, it 

 would be fairly entitled to be considered an organic deposit. 

 Though not finally established, the hypothesis of Sir Wyville 

 Thomson on this subject is of the greatest interest to the palae- 

 ontologist, as possibly serving to explain the occurrence, espe- 

 cially in the older formations, of great deposits of argillaceous 

 matter which are entirely destitute of traces of life. 



It only remains, in this connection, to shortly consider the 

 rock-deposits in which carbon is found to be present in greater 



