GEOLOGY. 



117 



Islands, their description would almost embrace that at our island. Of the 

 former Darwin says : — " Small well-rounded particles of shells and corals, of 

 white, yellowish, and piak colours, interspersed with a few volcanic particles." 

 Mr. Carne's description of the latter is identical, almost word for word. 



A similar instance of the comingling of volcanic particles takets place in 

 the calcareous sandstone of Rat and Booby Islands, two islets of the 

 Fernando do Norhona Group.* 



The Coral-sand rock consists almost wholly of carbonate of lime, four 

 analyses quoted by Mr. H. T. Wilkinson, giving an average of 75 per cent. 

 The following are the details : — 



Component Elements. 



1 2 



3 4 



1 



Carbonate of lime 



96 5 



Trace. 



3 5 



85-4 95.4 



96 4 



Phosporic acid 



Trace. 

 14-6 



Trace. 

 4-6 



Trace. 



Moisture, &c 



3-6 







Total 



100-0 



100-0 



100-0 



100-0 







The specific gravity is 2*452, that of ordinary limestone varying from 2*6 

 to 2-75. 



The consolidation of this rock is undoubtedly due to the percolation of 

 water, whereby carbonate of lime is dissolved, and redeposited on evapora- 

 tion, as a cementing medium, the agglutination probably going on rapidly. 

 This process is described by the late Professor J. B. Jukes, as taking place 

 at Raines' Inlet,t Great Barrier Reef; and excellently by Professor H. N. 

 Moseley at Bermuda. £ 



The stratification is usually very evident, and excellently shown at many 

 places along the shore, particularly in the section at Thompson's Point, the 

 laminae varying from one to three inches in thickness. At times, however, it 

 is difficult to distinguish either, from the manner in which large and small 

 masses have been tossed about, not, I think, by any convulsions of nature, 

 but simply by the undermining action of the waves and the faces of the 

 sections obscured by debris. The Coral-sand rock is particularly susceptible 

 to weathering, and it is even possible to distinguish in large blocks between 

 the effect produced by atmospheric and marine denudation. In the former 

 case very fantastic figures are sometimes produced, pinnacles, ledges, and 

 long reef -like floors and walls, the whole surface being eaten into a minutely 

 vesicular or honeycomb appearance. The inshore escarpments and isolated 

 patches, weather with a much more jagged and broken aspect, the honey- 

 comb appearance giving place to an open cavernous condition. When greatly 

 dessicated by either of these causes, all trace of lamination becomes lost,, 

 and this highly broken-up condition renders locomotion exceedingly difficult, 

 and, in the event of a fall, dangerous. Detached portions which have lain about 

 near the cultivations become rounded and waterworn, and the cavities filled 

 with the ordinary red soil of the island, when many strange outlines are pro- 

 duced. These irregular spaces vary from one to six and nine inches in diameter r 

 and perhaps even more. As a rule, when the Coral-sand rock rises in low cliffs 

 at and above high-water mark the foreshore is formed by an ordinary marine 

 platform, flat, with a seaward inclination. Such may be seen on the south 

 side of Prince William Henry Bay. On this Coral-sand rock platform the ba- 



* Moseley, Notes bv a Naturalist, 1879, p. 79. 

 t Voyage of the "Fly," 1874, n, p. 339. 

 t Notes by a Naturalist, 1879, p. 20. 



