26 INTRODUCTION. 



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 accumulate to form a calcareous ooze ; but at 

 greater depths they would be acted upon by the water, their 

 lime would be dissolved 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 decom- 

 position 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 palaeontologist, as 

 possibly serving to explain the occurrence, especially hi the 

 older formations, of great deposits of argillaceous matter 

 which are entirely destitute of traces of life. 1 



It only remains, in this connection, to shortly consider 

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

 greater or less quantity. In the great majority of cases 

 where rocks are found to contain carbon or carbonaceous 

 matter, it can be stated with certainty that this substance 

 is of organic origin, though it is not necessarily derived from 

 vegetables. Carbon derived from the decomposition of 

 > animal bodies is not uncommon ; though it never occurs in 

 such quantity from this source as it may do when it is 

 derived from plants. Thus, many limestones are more or 

 less highly bituminous ; the celebrated siliceous flags or so- 



1 Further investigations have thrown doubt upon the above theory, and 

 seem to favour the view that the red ooze is produced by the decomposition 

 of volcanic matter. 



