GROWTH OF GEOLOGY. 269 



once, and it was an important step which Ehrenberg 

 made in 1839, when, by applying the microscope, he 

 proved that chalk rocks were built up of the minute 

 shells of Foraminifera. The full importance of 

 this became plain when the Challenger explor- 

 ers mapped out the extent of Foraminiferal ooze 

 on the ocean floor. What is now accumulating in 

 the abysses was seen to be the modern analogue of 

 ancient chalk-cliffs, and the present-day represen- 

 tation of other than Foraminiferal limestone rocks 

 has also been disclosed. The Challenger Report 

 on Deep-Sea Deposits by Sir John Murray and the 

 Abbe Renard (1891) may be cited as the most im- 

 portant outcome of this line of investigation. 



The history of the study of coral-reefs, which we 

 have been forced to omit, is a very instructive in- 

 stance of gradually increasing thoroughness in the 

 investigation of a particular problem. 



The Living Earth. Until Charles Darwin fol- 

 lowed up Gilbert White's luminous suggestions and 

 made a careful estimate of the work of earthworms 

 as soil-makers, few naturalists even had any ade- 

 quate conception of the busy world beneath their 

 feet. Fifty-three thousand earthworms per acre, 

 bringing ten tons of soil per annum to the surface, 

 burying thousands of leaves and thus forming vege- 

 table mould, bruising the particles into fineness, and 

 by their burrows acting as ploughers before the 

 plough, facts like these, which Darwin substan- 

 tiated with his consummate patience, made it plain 

 that these humble creatures must be regarded as 

 among the most useful and important animals. 



But we must add details to our picture* of the 

 earthworms in their burrows; there are the moles 

 and the sharp-toothed centipedes both persecuting 



