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



Another specimen of augite-andesite contains zonary sections of felspar, parallel to 

 M, which allow the extinction to be measured accurately. They show that the plagio- 

 clase is labradorite (extinction 23°) at the centre, and bytownite (extinction 29°) on the 

 edges. The rock is altered on the surface, and covered with a whitish layer, to which 

 we shall return presently; the undecomposed portion contains 55 per cent, of silica. 



A specimen, which must be classed as basalt, presents just the same kind of surface 

 alteration into whitish material ; it has been so much decomposed by the action 

 of fumaroles that only felspar and a few grains of olivine can be distinguished. 

 Microscopical examination shows a number of large and sharply defined crystals of 

 olivine with the angles of this mineral and cleavages oo P co , OP. Augite has a 

 reddish tint, more common for this mineral in basalt than in augite-andesites, where 

 the colour is usually green. It occurs in large microporphyritic crystals, and is 

 often found as microliths in the ground -mass, frequently in small prisms forming 

 a zone round larger crystals of the same kind. The plagioclase crystals are twinned 

 according to the albite law, and sometimes according to that of pericline. Sections 

 showing both systems of lamellae very clearly, and almost parallel to h, give extinctions 

 from 30° to 35°, measured from the trace of M. This extinction angle classes this 

 felspar near labradorite. The ground-mass is that of an ordinary felspathic basalt. 



The action of fumaroles has so penetrated the specimen we are about to describe, 

 that, were it not for its density and structure, one might take it at first sight for a 

 fragment of pumice. Microscopically the alteration appears in the following manner : the 

 ground-mass is composed almost entirely of a quartzose aggregate, in which no well- 

 formed crystals are to be seen, but only grains of plagioclase and augite traversed in 

 every direction by cracks, the augite especially. Some remains of olivine crystals may 

 sometimes be seen. The rock is sprinkled with little brownish patches of a substance 

 occurring also crystallised in small prisms, the appearance and arrangement of which 

 strongly resemble sagenite ; but they are so small, so opaque, and so entirely surrounded 

 by the ground-mass, that it is impossible to determine their nature with certainty. 



XIV.— ROCKS OF THE PHILIPPINE ISLANDS. 

 A. Rocks from the Volcano of Camiguin. 



The island of Camiguin, on which the volcano about to be described is situated, 

 belongs to the Philippine archipelago, one of the most remarkable centres of eruption 

 on the globe. These islands form a link in the great volcanic chain, which, embracing 

 the Kuriles, Japan and Formosa, passes through Mindanao and Sangir, and runs out 



