REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 13 



II.— ROCKS OF THE CAPE VERDE ISLANDS. 



A. Rocks of St. Vincent. 



The archipelago of Cape Verde consists of eight large islands, two of which, St. Iago 

 and St. Vincent, were visited by the explorers of the Challenger ; they also landed at 

 Bird Island, one of the islets of the group situated near St. Vincent. We shall examine 

 first the rocks collected on the last-mentioned island, which is essentially of a volcanic 

 character, presenting an arid and desert aspect. The hills around Porto Grande are 

 formed of igneous rocks, of which each of the superposed beds do not attain a metre 

 in thickness. These sheets are slightly inclined, their dip increasing as they recede 

 from the port. They are frequently traversed by vertical dykes of basalt, of which the 

 general directions are N.-S. and E.-W. These injected basalts show a columnar 

 structure perpendicular to the sides of the rocks traversed. This prismatic structure is 

 also found in the beds of the rock constituting the principal mass of the hill. At the 

 contact of the intrusive rocks with the beds which they traverse, both are much 

 decomposed and disintegrated, — the latter being partially converted into a substance 

 resembling kaolin. As these veins traverse the hill from base to summit, and offer 

 more resistance to denuding agencies, they remain as walls of rock crowning the 

 heights with a jagged outline which is very characteristic. 1 



According to Professor Doelter, 2 the history of this volcano may be sketched as follows. 

 St. Vincent is the ruin of a strata- volcano of which the height was considerably greater 

 than the crest of that part of the crater now remaining. It is difficult to determine 

 the exact dimensions and the position of this ancient crater ; it appears, however, 

 probable that it must have been situated within an area at present comprising the port, 

 the undulating ground, and the plains which extend behind Porto Grande. Erosion 

 and the action of the waves have produced such profound modifications of the 

 surface, that it is scarcely possible to indicate exactly the original form of the 

 volcano. It appears to have been formed on a land surface of considerably greater 

 extent than the present island, as indicated by the hills formed of eruptive rocks of 

 ancient type (diabases, syenites), the age of which it is difficult to determine with 

 precision. On the south-west side of this great volcano, which is characterised by 

 sheets of lava, and occasionally by tufas traversed by numerous dykes, a consider- 

 able number of secondary craters have been formed, that do not appear to be of 

 ancient date. The presence of somewhat recent calcareous beds, which are spread out 

 on the slopes of Monte Viana and at other points, especially on the north shore, 



1 Buchanan, On geological work done on board H.M.S. Challenger, Proc. Roy. Soc, vol. xxiv. p. 612, 1876. 



2 Doelter, Die Vulkane der Capverden und ihre Produkte, p. 44, Gratz, 1882. 



