REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



63 



to crystalline rocks of the ancient type, and which have, as Darwin states, been torn up 

 from the depths by eruptions of basalt or trachyte. We shall describe them in detail, 

 commencing with those from Green Mountain, the locality of Darwin's specimens just 

 described. 



Amphibolic granites occur amongst the fragments brought to light by recent volcanic 

 masses. They resemble the granitic rocks we shall describe as enclosed in the augitic 

 andesites of Camiguin. The specimens are rather brittle, and are composed of vitreous- 

 looking grains. The felspathic mass is milk-white, dotted with the projecting black 

 points of little crystals of hornblende, which also line the walls of small geodes. To 

 the naked eye the rock presents the fritted appearance we will describe in speaking 

 of the granitic inclusions in the volcanic rocks of Camiguin. Under the microscope the 

 texture is distinctly granitoid ; numerous felspathic sections may be observed, and a few 

 of hornblende and quartz. The sections of felspar are often twinned according to 

 the Carlsbad law. The intercalation of plagioclastic lamellae, which do not fail to 

 appear in triclinic felspars, is not observable. The sections, however, do not show 

 the homogeneity of ordinary sections of orthoclase ; those parallel to the face M 

 are furrowed with little veins slightly expanded in the middle. These short veinules 

 are ranged in lines in the direction of the prismatic cleavage. On measuring the 

 angles of extinction on a section parallel to M, it is found that the principal individual 

 (that in which the veinules are imbedded) extinguishes at + 5°, the value of extinction 

 for orthoclase on this face. The spindle-shaped veinules, on the contrary, have an 

 extinction of the same sign, but much greater, the angle attaining 18°, the extinction 

 of albite. We may conclude that this fel- 

 spar is orthoclase, including fine lamellae of 

 albite (see fig. 12). This determination as micro- 

 perthite is again confirmed by the fact that we 

 have never been able to detect in any of the 

 felspathic sections the intercrossed lamellae of 

 microcline. The innumerable gas enclosures with 

 which the sections are riddled, giving a scorified 

 appearance to the mineral, seem to characterise 

 this felspar, and perhaps to indicate the high 

 temperature to which it was exposed during its 

 transport by the molten lava. With the excep- 

 tion of this the sections of felspar show only very 



slight traces of modification. Hornblende presents itself in irregular sections 

 are very pleochroic : 



Fig. 12. — Amphibolic granite. Section of orthoclase 

 with veinules of albite ranged in lines following the 

 prismatic cleavage (microperthite). fe crossed nicols. 



They 



almost black. 



y > 



dark green. 



brownish yellow. 



