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GEOLOGY OF FEBNANDO ffORONHA. 



91 





cemented together by carbonate of lime from decomposition of 

 shells, and on the top of San Jose we found some masses of a 

 hard quartzite-like sandstone, apparently formed of blown sand 

 and including a Corbula and a Venus, neither species met with 

 elsewhere. In this rock also was a pseudomorph of apparently 

 a felspar crystal, about an inch long, consisting of pure white 

 translucent quartz. There was not much of this rock, and it is 

 quite possible that it might not belong to the island. 



Salt crystallizes out here and there on San Jos6 Island and 

 elsewhere, from evaporation of sea-water ; and calcite coats the 

 raised coral-reef at Tobacco Point, and perhaps is the cementing 

 material which in some spots of Peak Bay forms the pebbles of 

 the beach into a conglomerate. It also lines and fills up cracks 

 and fissures in the softer basaltic rocks at Cotton-tree Bay and 

 in the phonolitic tuifs at Sponge Bay. 







General Summary. 



land 



been constan tly affirmed, of volcanic origin, and further that we 

 can trace two distinct periods in their history, the phonolitic and 

 the basaltic periods ; that the phonolite was ejected in the form 

 of phonolitic lavas and tuffs, and that there were periods of 

 cessation of action between the eruptions, during which some hot 

 spring deposited beds of silica. After this had happened, and 

 perhaps at a much later date, and after much denudation had 

 taken place, craters in the north-west and south-east portions 

 of the island ejected scorias, pumices, tuffs, and basalt, which 

 covered a great portion of the phonolitic rocks and altered them 

 where it came in contact with them. At a later period, probably 

 when volcanic action had ceased, portions of the submerged basalt 

 beds became covered with a thick deposit of coral-reef, sometimes 

 100 feet in thickness, which was perhaps gradually raised above 

 sea-level to a height of from 3 to 4 to 100 feet, and which, in some 

 spots, having been used by the sea-birds for many years as a 

 resting- and probably as breeding-places, became covered with a 

 deep layer of guano, on which a rich vegetation soon established 

 itself. All volcanic action seems to have ceased for many years, 

 there are no traditions of tidal waves or earthquakes, and the 



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