The Bocks of Tristan d'Acunha. 25 



needles are found enclosed in all the minerals of the rock, and 

 among the secondary minerals, are magnetite quartz, arfedsonite, 

 and a blue soda-amphibole. 



Prof. Cole, in describing specimens of boulders dredged off the 

 Eockall Bank, states that they mostly consist of basalt, pumice, 

 and scoriaceous andesite ; one specimen, however, was a grey 

 micaceous sandstone, another a red sandstone ; nothing could 

 be said respecting these, except that they resembled Torridonian 

 and Ordovician rocks, so that the stratified beds on Eockall Island 

 cannot yet be determined/" The point of greatest interest in the 

 Eockall Island is that, although it is not truly an oceanic island, yet 

 lying as it does so far from the mainland, it still contains rocks 

 of the continental types, and one must suppose that similar ones 

 are to be found under the sea beyond it. 



In the West Indies we do not get truly oceanic islands, and yet in 

 the smaller islands no rocks of a continental type have been recorded.! 

 In nearly all the larger ones, however, crystalline, schists, sand- 

 stones, and shales are found. Trinidad is simply a prolongation of 

 the continent, the channel which separates it from the mainland 

 being only 36 feet deep, yet it is as certain as anything can be in a 

 question of this sort that the same ridge that carries Trinidad, 

 Puerto Eico, Haiti and Cuba, and comes up again at Yucatan on the 

 one hand, and Florida on the other, is continuous throughout, and 

 that the string of volcanic islands, forming the lesser Antilles, stands 

 on a base made up of the ordinary types of continental rocks. 



Before leaving the Atlantic, it is worth noting the persistency 

 of the rumours as to land having once existed in mid-ocean. 

 Plato called this land Atlantis ; Aristotle, Antilla ; while northern 

 nations knew it under the name of Brasil. The extraordinary habit 

 of lemmings in Norway and Sweden, at certain periods collecting 

 together and making due west till they came to the sea, and, not 

 stopped by this, of swimming, swimming ever westwards till they are 

 all drowned, has been explained by supposing that at some distant 

 date they used periodically to migrate to some land now submerged. 



Off the Canaries, also, a mysterious island has been known in 

 legend from the earliest times. Pero Diaz, a monk of the holy 

 order of St. Francis, is said to have seen it in 1759, lying to the 

 west, and having the appearance of the Blessed St. Anthony playing 

 on a dulcimer ; Fernando Correa, fisherman, who saw it at the same 

 time, likened it to the head of a mule playing on a flute ; while 



* Trans. Koy. Irish Acad., vol. xxxi, 1897, pt. 3, pp. 39-98; extract in 

 Geological Magazine, Dec. iv., vol. vi. No. 418, p. 163, April, 1899. 

 t Spencer, West Indian Papers, Q.J.G.S., lviii. , 1902. 



