THE FOSSILIFEROUS ROCKS. 36 
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 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 pale- 
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 carbo is found to be present in greater 
