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 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 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.