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



(the angle of some labradorites) in sections parallel to M, and the internal part at an 

 angle of 10°. Sections perpendicular to the edge PjM extinguish at 20° for the central 

 part, and at 30° for the outer zone. These observations confirm our previous statements, 

 that the central crystal is andesine, the enveloping pellicle labradorite (see fig. 11). We 

 may add that many of the crystals, even the microliths of the ground-mass, show the 

 Carlsbad twin. The smallest plagioclastic microliths have the extinction of labradorite, 

 the second generation of felspar is then more basic than the first. 



Ejected Fragments of Amphibolic Granite, Granitite, Diabase, and Gabbro. 



Darwin 1 observed heterogeneous fragments of rocks included in the scoriaceous 

 volcanic masses of Green Mountain, and his description of these may be recalled here. 

 Nearly all the specimens had a granitic structure ; they crumbled readily, were rough 

 to the touch, and their original colour was altered. Darwin classed these fragments, 

 and grouped them as follows : — 



1. A whitish syenitic rock, striped and spotted with red markings. Felspar is well 

 cr3 T stallised, and numerous small brilliant grains and crystals of quartz are visible. The 

 felspar and hornblende were determined by means of the reflecting goniometer, and the 

 former mineral appeared from its cleavages to belong to a potash felspar. The quartz 

 was determined by the blowpipe. 



2. A fragment of a brick-red colour, composed of felspar, quartz, and dark particles 

 of an altered mineral, which appears from its cleavages to be hornblende. 



3. A mass of whitish felspar crystallised in a confused manner and containing small 

 cavities filled with a decayed mineral, dark in colour, with rounded edges, shining 

 fracture, but no definite cleavage plane. Comparison with the preceding specimen 

 justifies the conclusion that it is fused hornblende. 



4. A rock which appears like an aggregation of large crystals of dark-coloured 

 labradorite, amongst which granules of whitish felspar, numerous micaceous lamellae, 

 and altered hornblende are found, but quartz is absent. 



Darwin states also that he picked up at another point a conglomerate containing 

 small fragments of granite, of cellular or jasper-like rocks, and of porphyry, enclosed in a 

 wacke traversed by many fine threads of concretionary pitchstone passing into obsidian. 

 These beds are parallel, gently undulating ; they continue for only a short distance, 

 thinning out at the extremities like the lenticular enclosures of quartz in gneiss. He 

 adds that it is possible that these fragments were not thrown out separately by the 

 volcano, but that they were brought to light enclosed in a fluid mass resembling 

 liquid obsidian. 



Amongst the specimens we have examined there are several which may be referred 



1 Darwin, Geol. Obs., pp. 40-42. 



