164 THE VOYAGE OF H.M.S. CHALLENGER. 



on slightly undulating and greatly denuded strata, formed, as can be seen on the shore, 

 of beds resembling trachyte. We shall now describe the lithological nature of the 

 eruptive products that constitute the volcano. 



The rocks collected at Camiguin belong to the andesite type ; sometimes, as we shall 

 show, augite predominates in them ; in other instances hornblende seems to play the 

 leading part, but, in all cases, these two bisilicates are present, and the transition 

 between the amphibolic and pyroxenic andesites is gradual. We shall therefore 

 describe both types together. In general, these rocks are very close grained ; the 

 constituent minerals are readily detached from the mass ; the colour is greyish passing 

 into reddish on alteration ; when the rock is more massive, it is a little darker. With 

 the naked eye or the lens it is possible to distinguish only some whitish glassy grains, 

 which are plagioclases ; blunted crystals of black hornblende, or patches of augite 

 approaching a greenish tint, are sometimes seen. 



Microscopical examination shows that these rocks belong to two types of andesites, 

 the amphibolic and the pyroxenic, passing from the one to the other through all 

 stages ; in some instances, by the presence of olivine, they are allied to the basalts. 

 In all, however, the microtexture and mineralogical composition remain much the same. 

 In a ground-mass, composed chiefly of small prismatic crystals of plagioclase and augite, 

 united nearly always with a colourless glassy base, are embedded large fragments of 

 plagioclase, augite, generally in greenish grains, hornblende without any crystallo- 

 graphic outlines and of a yellowish brown hue ; and, lastly, biotite, bronzite, and 

 especially magnetite, which is scattered in small sections everywhere, both in what we 

 call the paste and in the sections of the above-named minerals. 



Having now indicated the microscopical texture and the constituent minerals, 

 we shall describe the characters which each of them presents under the micro- 

 scope. Plagioclase is incontestably the most important and interesting mineral in the 

 andesites of Camiguin. The adjoining figures represent some of the sections of these 

 felspars. 



The group represented in fig. 30 shows two individuals 

 twinned according to the albite law. The principal indi- 

 viduals are joined following M; one observes the repetition 

 of I and II reciprocally intercalated in each of the two 

 individuals. In the lower part of the figure, the reentrant 

 angle a is formed by the traces of P of I and II. In the 

 _, on , . ." ,„ . . upper part the obtuse angle is 7° 50'. The double angle 



Fig. 30.— Andesite of Camiguin. £ 7 . . „ ° , 



section of plagioclase, albite of extinction is 70° (32°-38°) ; 7 indicates intercalation 



of lamellae following the pericline law. The intercala- 

 tion of these lamellae shows that the section is very nearly perpendicular to the 

 edge P/k. 



