688 NOTES TO BOOK XVIII. 



country we ought probably, with Mr. Darwin, to include 

 the " parallel roads" of Glenroy, the subject, in former 

 days, of so much controversy among geologists and anti- 

 quaries. 



Connected with the ^ecular rise and fall of large por- 

 tions of the earth's surface, another agency which plays an 

 important part in Geological Dynamics has been the sub- 

 ject of some bold yet singularly persuasive speculations 

 by Mr. Darwin. I speak of the formation of coral, and 

 coral reefs. He says that the coral-building animal works 

 only at small and definite distances below the surface. 

 How then are we to account for the vast number of coral 

 islands, rings, and reefs, which are scattered over the 

 Pacific and Indian Oceans ? Can we suppose that there 

 are so many mountains, craters, and ridges, all exactly 

 within a few feet of the same height through this vast por- 

 tion of the globe's surface ? This is incredible. How then 

 are we to explain the facts ? Mr. Darwin replies, that if 

 we suppose the land to subside slowly beneath the sea, and 

 at the same time suppose the coralline zoophytes to go 

 on building, so that their structure constantly rises nearly 

 to the surface of the water, we shall have the facts ex- 

 plained. A submerged island will produce a ring ; a long 

 coast, a barrier reef ; and so on. Mr. Darwin also notes 

 other phenomena, as elevated beds of coral, which, occurring 

 in other places, indicate a recent rising of the land ; and on 

 such grounds as these he divides the surface of those parts 

 of the ocean into regions of elevation and of depression. 



The labours of coralline zoophytes, as thus observed, 

 form masses of coral, such as are found fossilized in the 

 strata of the earth. But our knowledge of the laws of life 

 which have probably affected the distribution of marine 



