CORAL ISLANDS. THE ABYSSES OF THE OCEAN. 45 



tralia. Thus Darwin's theory explained the form and 

 the approximate identity of. altitude of these coral 

 islands. But it did more than this, because it showed 

 us that there were great areas in process of subsidence, 

 which though slow, was of great importance in physical 

 geography. 1 



Much information has also been acquired with refer- 

 ence to the abysses of the ocean, especially from the 

 voyages of the Porcupine and the Challenger. The 

 greatest depth yet recorded is near the Ladrone Islands, 

 where a sounding of 4,575 fathoms was obtained. 



Ehrenberg long ago pointed out the similarity of 

 the calcareous mud now accumulating in our recent 

 seas to the chalk, and showed that the green sands of 

 the geologist are largely made up of casts of Foramini- 

 fera. Clay, however, had been looked on, until the 

 recent expeditions, as essentially a product of the disin- 

 tegration of older rocks. Not only, however, are a 

 large proportion of siliceous and calcareous rocks either 

 directly or indirectly derived from material which has 

 once formed a portion of living organisms, but Sir 

 Wyville Thomson maintains that this is the case with 

 some clays also. In that case the striking remark of 

 Linnaeus, that ' fossils are not the children but the 

 parents of rocks,' will have received remarkable confir- 

 mation. I should have thought it, I confess, probable 

 that these clays are, to a considerable extent, composed 

 of volcanic dust. 



It would appear that calcareous deposits resembling 

 our chalk do not occur at a greater depth than 3,000 



1 I ought, perhaps, to mention that Darwin's views have been recently 

 questioned on some points by Semper and Murray. 



