26 THE VOYAGE OF H.M.S. CHALLENGES. 



regions, form massive ridges on which the volcanic products of the islands of these 

 seas have spread themselves. 1 



An obvious as well as cogent argument in favour of an eruptive origin is afforded by 

 the fact that the bottom of the Atlantic has been for long ages, on many points, the 

 theatre of volcanic manifestations ; and, in particular, the region in which the rocks of 

 St. Paul are situated, has, in comparatively recent times, shown signs of eruptive 

 phenomena. For more than a century, indeed, appearances have been observed in 

 this very region that prove it to be the centre of volcanic activity. Scrope 2 says that 

 Daussy, 8 and after him Darwin, 4 have brought together a number of traditions that 

 indicate the existence of a vast volcanic region beneath the Atlantic, midway between 

 Cape Palmas, on the West Coast of Africa, at lat. 4 N., long. 10 W., and Cape St. Eoque 

 in South America, lat. 50 S., long. 37 37' W., in the narrowest of the ocean belts, 

 at the central point of which the island of St. Paul is situated. 



The isolation of these rocks might be adduced as a further proof of their eruptive 

 origin. The soundings made between St. Paul and the nearest continent and other 

 islands, tend to show that the rocks possesses a purely local character in perfect harmony 

 with the theory of volcanic formation. 5 



Having thus exposed the reasons which appear to militate in favour of the first 

 hypothesis (the volcanic origin), let us now consider what may be advanced to support 

 the idea that this peridotite might belong to the schis to-crystalline series. Since the 

 arguments in favour of the eruptive origin are not absolutely conclusive, it naturally 

 follows that the second alternative may be maintained. Indeed, we have seen that most 

 peridotites belong to the schisto-crystalline series, and we cannot separate their mode of 

 formation from that of the rocks with which they are associated. In the peridotite 

 of St. Paul the banded structure, the position assumed by the crystals in the mass, their 

 form, in short, all the peculiarities above mentioned, are characteristically those of the 

 schists. If this, after all, be the true position of the rock, as we are rather inclined 

 to believe, how are we to account for the presence of such a peridotic mass isolated in 

 mid- Atlantic? 



1 Cohen (Ueber die sogenannten Hyperstenite von Palma ; Neues Jahrbuch, 1876, p. 747) describes some of 

 the rocks of the island of Palma as either covered with, or cut through by, more recent lavas. He considers them as pre- 

 tertiary. He did not find any hypersthenite, and he classes them as diabases, olivine-diabases, diorites, and syenites, 

 &c. Van Werveke has recently described a limburgite from the same island (Neues Jahrb., 1879, p. 482). Finally, 

 S. Calderon, in a paper read before the Geol. Soc. of London in June 1879, distinguishes two types of rocks at Palma ; 

 the one, more ancient, characterised by the presence of hornblende, the other, more recent, by that of augite. 



2 Scrope, Volcanoes, p. 237. 



3 Daussy, Note sur 1'existence probable d'un volcan sous-marin situe par 20' lat S. et 22 long. 0., Comptes 

 rendus, 1838, Avril, p. 512. 



4 Darwin, Geological Observations on Volcanic Islands, 1844, p. 92. 



5 The Challenger soundings and temperature observations show a low ridge running the whole length of the 

 Atlantic, with about 2000 fathoms over it It is called " Dolphin" ridge in the North Atlantic ; " Connecting Ridge " 

 at the Equator ; " Challenger" ridge in the South Atlantic ; and on it are situated Tristan d'Acunha, Ascension, St 

 Paul's Rocks, and Azores. J. M. 



