378 AUSTRALIA : 



similar but shorter one on the western coast of New Cale- 

 donia. The term, however, has been fitly extended by Mr. 

 Darwin to those reefs encircling smaller islands, which are so 

 numerous in the Pacific, and familiar to us in the narratives 

 of voyages in this ocean ; coral walls in fact, with a deep 

 moat within, circling round islands of every various dimen- 

 sion and height ; some little raised above the sea others 

 like Tahiti, having an elevation of many thousand feet. 



' Fringing or Shore reefs,' whether encircling islands or 

 portions of continents, differ from those just described in 

 being less massive, in having no interior deep-water channel, 

 and in sloping downwards into the sea upon the natural de- 

 clivity of the shores. The reefs of the Mauritius furnish a 

 well-marked insular example of them. The coasts of Brazil 

 and Arabia afford instances, among many that might be 

 quoted, of such coral fringes to continental lands. 



Mr. Darwin has done much to simplify the view of the 

 several coral formations just noted, by showing that they 

 graduate into one another ; and that the atolls, barriers, and 

 encircling reefs are but modifications, deviating much in the 

 extreme cases, of a common principle and manner of opera- 

 tion. A perfect series, in fact, can be traced from the simple 

 linear or normal state of the reef, to the long linear lagoon, 

 and thence to the oval or circular form of the encircling reef 

 or the atoll. Again, if from the barrier reefs encircling small 

 islands we abstract the land within (a legitimate specula- 

 tion, as will hereafter be seen), we bring them to an almost 

 complete identity with the simple atoll or lagoon island, in 

 form, dimensions, and grouping. The value of such gene- 

 ralisations as these to a just theory on the subject will be 

 well understood, even without a full comprehension of the 

 details on which they are founded. 



The fourth chapter of Mr. Darwin's volume relates to the 

 distribution of coral formations, and their rate and manner 



