GEOLOGY. 



117 



Islands, tlieir iloscriptioii would almost embrace tliat at our island. Of the 

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

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

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



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

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

 Fernando do Norhona G-roup.* 



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 





96-5 



Trace. 



3-u 



85-4 



Trace. 



14-6 



95.4 



Trace. 



4-6 



96-4 



Pliosporic acid 



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-G 

 to 2'75. 



The consolidation of this rock is undoubtedly due to tlie jjercolation of 

 ■tvater, 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 Eaines' Inlot,t Groat Barrier Ecef; and excellently by Professor II. N. 

 Moseley at Bermuda. J 



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

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

 laminge 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 obscu.redby dehris. The Coral-sand rock is particularly susceptible 

 to weathering, and it is even possible to distinguish in large blocks be-tween 

 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 suri'ace 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 app)earance 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 watcrworn, 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 ^^ 

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

 at and above high-water marlv 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. 

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

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



79. 



