18 THE VOYAGE OF H.M.S. CHALLENGER 



B. Rocks of St. Iago. 



St. Iago is one of the most remarkable islands of the Cape Verde archipelago. It 

 was explored by Darwin l during the voyage of the " Beagle," and more recently by 

 Professor Doelter. 2 Our observations having been confined to a few specimens collected 

 near Porto Praya, we shall restrict ourselves to the description of these rocks, referring 

 for further information to the works of Darwin and Doelter. We shall merely state 

 that the part of the island where the rocks were collected of which we are about to give 

 the analysis, constitutes a natural division of St. Iago, 3 — a plateau which stretches 

 from Pico d'Antonio to the sea. This plain is formed of lavas slightly inclined, and 

 pierced by more recent eruptions. The thickness of the lava varies from 300 to 900 

 feet, each sheet having a thickness of 30 to 45 feet; the layers are separated by 

 rather thin intercalations of tufa. In this part of the island a bed of limestone may be 

 observed ; it is of recent formation, for it contains shells now living in the surrounding 

 sea. The ancient lavas of Pico d'Antonio are anterior to this limestone, which contains 

 fragments of them. 



Amongst the rocks collected near Porto Praya are to be mentioned, in the first 

 place, specimens which may be referred to limburgite. They are of a reddish grey 

 colour, with numerous vesicles in which natrolite has crystallised. Under the micro- 

 scope it is seen that this rock contains an abundant, brownish, vitreous base, and is 

 transformed, along the veins and fissures, into a reddish substance which may be 

 observed in rocks of the basaltic series undergoing modification into palagonite. 

 In this base are observed pretty large and remarkably well outlined sections of olivine. 

 This mineral is little if at all altered, and the only inclusions observed in it are crystals 

 of magnetite. Several crystals are frequently joined with parallel axes ; the sections in 

 this case show outlines with re-entering angles ; but in many cases it may be ascer- 

 tained with polarised light that these crystals are not twinned, but simply juxta- 

 posed. There are others, however, in which the phenomena of polarisation show 

 that the axes of elasticity are oriented so as to render the existence of a twin 

 highly probable. The two crystals are joined at an angle of about 45° or 50°, but 

 the irregularity of the contours does not allow it to be measured with precision. 

 If these crystals are examined with convergent light, there may be observed on one 

 of them a bissectrix, indicating the plane of an optical axis perpendicular to the 

 long edge. On the other may be already observed the lemniscates, and an arm of the 

 hyperbola oriented in the same way. The observations make it sufficiently probable 

 that the two crystals may be twinned, with a dome as composition plane. 



1 Darwin, Geological Observations on Volcanic Islands, pp. 1-22, ed. i., London, 1844. 



3 Doelter, Die Vulkane der Capverden. 3 Doelter, loc. cit., p. 44. 



