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



individual, and show themselves in the manner we have described. The sections of 

 augite and olivine are in no way remarkable, except in being often corroded by the 

 magma. Augite frequently occurs as inclusion in plagioclase. We may also mention, 

 amongst the constituents of the rock, grains and crystals of magnetite, and a rounded 

 fragment of hornblende surrounded by a large zone of magnetite. 



Layers of volcanic conglomerate were observed near the fishermen's huts. The 

 microscope showed this rock to be made up of basaltic lapilli, and more or less frag- 

 mentary minerals, with rather vague outlines, embedded in a light greenish mass. In 

 the yellowish vitreous lapilli there are microliths of augite and small crystals of olivine. 

 Plagioclase is not so common as the former minerals, but appears sometimes in the 

 form of skeletons forked at both extremities. 



A limburgite coming from the bed of a river in Corinthian Bay deserves 

 description. This rock is greyish black, and the constituents are large enough to be 

 recognised by the naked eye as crystalline grains of olivine and augite. The micro- 

 scope proves the absence of felspar, and shows the ground-mass to be a brownish glass, 

 enclosing crystals of olivine and augite. The forms assumed by olivine in this rock 

 may be deduced from the microscopic sections. The hexagonal sections prove the 

 existence of faces of the prismatic zone surmounted by a face of a sharply pointed 

 dome. The angle between the traces of the dome is from 79° to 80°, and the value 

 of kjk is 80° 53'. The sections are grooved with cleavages at right angles, parallel 

 to the outlines of traces of the prism and to the base. The form of sections with 

 a reentrant angle shows that the olivine is often formed by juxtaposition of a certain 

 number of crystals with parallel axes. They are often corroded by the magma. The 

 examination of this rock tends to confirm an observation often made before in lim- 

 burgites, that the best developed element in this lithological type is olivine ; the 

 augite is often in the form of microliths embedded in the vitreous mass. Another 

 specimen of limburgite from Corinthian Bay, identical in composition and texture 

 with the preceding, is somewhat rich in zeolites, as this kind of rock nearly always is. 



The cliffs of the island contain layers of more ancient eruption. We have examined 

 some specimens of these ; they are greyer in colour and less scoriaceous in appearance 

 than the rock last described. In one fine-grained mass the lens showed the fel- 

 spathic element to predominate over the other constituents, and this was confirmed 

 by microscopic examination. This rock is a basalt like all those of Marion Island. 

 Microscopic preparations show large irregular or rounded sections of olivine and very 

 numerous lamellar plagioclases, between which are embedded small irregular grains 

 of augite. Magnetite occurs between the other constituents, and there are also a few 

 small scales of biotite. 



