REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



155 



observed between two hemitropic lamellae in the zone P :k are 32°, 21*, 19°, but in 

 some cases they exceed 35°. These values agree with the determination just given. 

 We see another confirmation of this in the fact mentioned above, of the rarity of 

 hemitropic plagioclastic striae ; it is well known that the extremes of the plagioclastic 

 mixtures, albite and anorthite, are to a certain extent characterised by the rarity 

 of these interpositions, or by the relative thickness of the hemitropic lamellae. 



The augite presents no very special characters ; it exhibits a tendency to form more 

 or less irregular groups or nests, and is often twinned. The very rare sections of 

 olivine, often occurring as inclusions in the plagioclase, are decomposed into red hematite. 

 Magnetite is somewhat abundant. The microliths of the ground-mass, as observed 

 above, are small crystals of plagioclase and augite, the former being often split up at the 

 extremities. 



The remaining rocks from the summit of Goonong Api have been altered by the 

 action of fumaroles, as in the case of certain lavas from Ternate, but in those from 

 Goonong Api decomposition is further advanced, and presents some phenomena worth 

 describing. These lavas have the same aspect and the same lithological constitution 

 as those just described, only they are much more friable, and covered in some places by 

 a floury coating. One sees with the lens that the felspar crystals have lost their glassy 

 lustre and appear porcellanous. Under the microscope the large sections of felspar show 

 hardly any remaining trace of the original twinning, but their outlines are maintained 

 notwithstanding the alteration that has destroyed the internal structure of the mineral. 

 The sections are furrowed with a lacework of cracks lined with 

 a colourless substance, in the same way as serpentinisation 

 penetrates olivine. A few patches of the original mineral 

 remain unaltered, but as a rule the entire section behaves 

 between crossed nicols like an isotropic substance. The 

 plagioclastic sections invaded by this secondary product 

 rarely show the twins of plagioclase, one can only detect 

 certain remains that react feebly with polarised light. These 

 crystals often appear cracked (see fig. 28). The first explana- 

 tion that offers itself to account for this strange phenomenon 

 of decomposition is that the rock, being formed of anorthite — 

 a plagioclase which lends itself very readdy to the formation 

 of zeolites — the alteration of the felspar would be due to a 

 modification of this kind ; but chemical analysis proves that 

 the substance penetrating the felspar is silica. In fact, the 

 undecomposed augite-andesites of Goonong Api contain from 

 55 to 59 per cent, of silica, and when they exhibit the alteration which has been 

 described the percentage of silica rises to 80 per cent., and, in the specimens trans- 



Fig. 28. — Lava of Goonong Api. 



Decomposed plagioclase partly 

 replaced by silica. 



