59G 



FORMATION OF CORAL REEFS. 



[Ch. XLIX 



flowing of lava, and tlie ejection of volcanic aslies, wonld 

 combine with tlie upheaving force to form new land. 



600 miles in length, to sink 15 feet, 



' Suppose a shoal, 



unmoved 



for a thousand years; during 

 that interval the growing coral may again approach the 

 surface. Then let the 



mass 



jU 



case 



the original reef is restored to its former position : in this 



the new coral formed since the first subsidence will 

 constitute an island 600 miles long. An analogous result 

 would have occurred if a lava-current 15 feet thick had 



^d reef. The absence, therefore, of 



submei 



tD 



seems 



that the amount of subsidence by earthquakes exceeds, in 

 that quarter of the globe, at present, the elevation due to the 



same cause. 



5^ 



me m 



from the struc- 

 the following 



Another proof also of subsiden* 

 ture of atolls, was pointed out 

 passage in all former editions. * 

 the Pacific,' says Captain Beechey, ' follow one general rule 



The low coral islands of 



more 



the other. 



Matilda 



is very conspicuous, the weather side of both being wooded, 

 and of the former inhabited, w^hile the other sides are from 



20 to 



under 



perceived to be equally narroiv and well defined. It is on 

 the leeward side also that the entrances into the lagoons 

 occur; and although they may sometimes be situated on a 

 side that runs in the direction of the wind, as at Bow 

 Island, yet there are none to windward.' These observations 

 of Captain Beechey accord with those which Captain Hors- 

 buro'h and other hydrographers have made in regard to the 



coral islands of other seas. From 



fortunate circum- 



stance ships can enter and sail out Avith ease ; w^hereas if 

 the narrow inlets were to windward, vessels which once 

 entered might not succeed for months in making their w^ay 

 out again. The well-known security of many of these 

 harbours depends entirely on this fortunate peculiarity in 

 their structure. 



* See Principles of Geology, 1st. edit., vol. ii. p. 296. 



i 



^ 



