REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 131 



Other specimens of much-altered whitish trachyte readily fall into powder. They 

 are as light as pumice, but of closer texture ; they greatly resemble the preceding rock, 

 except that the light vesicular vitreous matter, passing into a pumiceous structure, 

 plays a more considerable part. Crystals of plagioclase, intimately associated with 

 sanidine and apatite, may be mentioned as accidental elements. 



These trachytes are accompanied by reddish pumiceous trachytic tufas. Irregular 

 fragments may be seen by the naked eye embedded in a slightly scoriaceous paste. 

 Thin slices show that these tufaceous rocks are composed of a greyish mass, which is 

 isotropic in some places, and almost everywhere impregnated with iron. The little 

 fragments of rock enclosed in this grey mass are trachytic ; sanidine is the principal 

 constituent in them, associated with green microliths of augite. There are also large 

 splinters of very clear sanidine, which might be taken for cpiartz if they were not 

 biaxial ; finally, one observes large cracked crystals of green augite. 



As everywhere else in Kerguelen, basaltic rocks occur at Coronet Hill, but here they 

 are not very distinctly characterised. The specimens we class as basalts are scoriaceous, 

 very vesicular, with drawn-out pores ; in colour they are deep red, and nothing except 

 lamellae of black mica can be seen by the naked eye. Under the microscope the 

 ground-mass appears almost opaque from the interposition of a black pigment, with 

 numerous small green crystals of augite, and regular sections of olivine altered into 

 hematite. Large fragments of augite, sometimes enclosing hornblende, also appear. 



We have now to describe the rocks of Greenland Harbour. This fjord is situated to 

 the south of Royal Sound, from which it is only separated by a narrow tongue of land. 

 We may first recall the observations made by Mr. Buchanan in this part of the island. 

 On entering Greenland Harbour he was struck by the appearance of the masses of grey 

 rock which rise up boldly from the horizontal beds of basalt. The chain of hills near 

 this fjord is composed of basalt, the greatest mass of grey rock being found on the 

 summits in the western part of Greenland Harbour, and appearing from a distance 

 like a heap of ruins. He was able to examine this rock in two places, at the 

 summit of the hills west of the bay, and near the creek where he landed. He found 

 the rock to be the same on both sides ; it is a phonolite of a light greyish green colour, 

 surrounded by basalt. The masses of phonolite are cylindrical and columnar on the 

 outside, the columns being horizontal, and showing a radial arrangement. They do not 

 penetrate the rock, but form a zone some feet thick around the central part, which 

 remains massive. The prisms have been largely disintegrated by weathering, and lie 

 broken up into a great number of blocks around the phonolite masses. The outer 

 portion of the rock, in which the columns are horizontal, resembles a cyclopean wall, 

 and resists atmospheric agencies much better than the solid centre. Were it not for 

 these natural walls binding the whole mass together, the central part would form a 



