REPORT ON THE PETROLOGY OF THE ROCKS OF ST. PAUL. 3 



volume." J I am inclined to think that the passages just quoted have served as the 

 chief, if not the only ground for considering the rock-mass of St. Paul as belonging to 

 some continent now submerged. 2 



The mineralogical composition and microscopic structure of these rocks are no less 

 interesting to the petrographer than the mode of their formation is to the geologist. As 

 I shall presently show, they are almost exclusively composed of granular olivine ; they 

 may be even considered as one of the most remarkable types of olivine rocks recently 

 brought to light. 



Previous to the researches proving the existence of rock-masses essentially composed 

 of olivine, the presence of this mineral had been observed in basalts, dolerites, melaphyres, 

 and some other igneous rocks ; it was likewise found in granular limestones, talc-schists, 

 &c., &c. But it is only since the researches of Damour and Des Cloizeaux on the lherzolite, 

 of Hochstetter, on dunite (a compact olivine rock of New Zealand), that the peridotites 

 or rocks formed essentially of olivine were acknowledged as constituting a petrographic 

 type. 



Eesearches made within the last few years have shown that these rocks are of more 

 frequent occurrence than was at first supposed ; and we have now in the specimens from 

 St. Paul a type of peridotite remarkable for the purity of its composition, and the very 

 slight alteration it has undergone. 3 



Before entering on the lithological description of the rocks I will briefly describe 

 their position, borrowing chiefly from the information supplied by the preliminary 

 reports of the Challenger Expedition. 



The island of St. Paul is situated very nearly under the Equator, being in lat. 

 0' 58" N., and long. 29° 15' W. It is about 540 miles distant from the coast of South 

 America, and some 350 miles from the island of Fernando Noronha, about half way 

 between the coasts of Africa and of South America. 



The group of rocks consists of five peaks, divided into four principal masses, separated 

 by three narrow channels. In shape they resemble a horse-shoe, and stand round a 

 basin, or bay, which they enclose. Towards the base the rocks are black, but their 



1 Darwin, Volcanic Islands, p. 32. 



2 Boue, loc. cit., p. 12. 



3 It is not the first time that the presence of an olivine rock, more or less altered, has been observed in the Atlantic 

 islands. Darwin, in his Volcanic Islands, p. 18, states that at St Jago and Quail Island, Cape Verde, he found 

 considerable masses of wacke, which he describes as a pale yellowish green argillaceous substance, of a crumbling 

 structure when dry, but unctuous when moist. In its purest form it presents a beautiful green tint, with translucent 

 edges ; it has occasionally obscure traces of an original cleavage. It seemed to him, at first, that this wacke came 

 from the decomposition of an augitic mineral. This opinion was founded upon its pyrognostic characters, and the 

 presence of augite in the basalt associated with this green material. However, having examined the stages of decom- 

 position of the olivine contained in the basalt, he found that he could trace a nearly perfect gradation from the 

 unaltered olivine to the green material he called wacke. The pyrognostic characters served to confirm this opinion. 

 What struck him most forcibly was, that a mineral considered infusible should yield by decomposition a product 

 perfectly fusible. 



