REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 179 



sections of olivine are to be found in this fundamental mass, among which no glassy 

 matter is seen. This mineral plays an important part as a porphyritic element ; it is 

 found in the microscopic preparations in large sections with usually rounded angles, 

 bordered by a zone coloured yellow with hydroxide of iron ; this zone follows exactly 

 the outlines of the crystals, and lines all their crevices. Sometimes three or four 

 crystals of olivine are grouped together, often several individuals are coupled with their 

 vertical axes parallel. A striking characteristic of these sections is that they present 

 two equal rectangular cleavages, which, at first sight, makes them look like sections of 

 augite ; the cleavage parallel to the face ooPoo is generally observed, but the cleavage 

 parallel to coPoo is here as clearly marked. Several sections of olivine, with hexagonal 

 outlines, are ended by an obtuse dome of about 103° ; these sections must be parallel 

 to a face of the prism, for an optic is seen exactly in the centre of the field. The long 

 sides of such a section are traces of the faces of the prismatic zone (prism or pinacoid). 

 The angle of the summit does not correspond with the dome P oo nor with P oo ; it 

 must be therefore ascribed to a pyramid. This face of a pyramid more lowered than 

 the aforesaid domes forms the obtuse angle so often observed in the olivine of basaltic 

 rocks. 



The rocks near the monument erected to Selkirk's memory are of the same character 

 as the dolerites and basalts just spoken of. These specimens have the same appearance 

 as the basalts with large crystals of olivine, but this mineral is not visible with the 

 naked eye, the rock is more vesicular ; with the microscope it is seen that the texture 

 of this rock is more like that of a dolerite. The lamellae of plagioclase are very narrow 

 as in the former case, symmetrical extinctions have given almost an angle of 30°. The 

 augite is moulded on the other constituent minerals ; sometimes it is to be observed 

 with the clepshydron structure ; it appears in the fundamental mass in the shape of 

 grains. Sometimes the augite is macroscopic, and seems to take the place of olivine. 

 The latter is again to be observed in sections with an obtuse top ; this mineral is 

 bordered by a zone of hydroxide of iron. A vein of limonite runs through the whole 

 of the microscopic slides. Viridite has been deposited in some spots. 



Among the specimens collected on the coast of Juan Fernandez it is necessary to 

 mention a greyish very scoriaceous rock, from which stand out large crystals of plagio- 

 clase, of waxy and milky appearance, lengthened following the edge PjM. This rock 

 is a dolerite with large vesicles, the only difference between it and the formerly 

 described rocks being in its structure. Under the microscope the fundamental mass, in 

 which the plagioclase crystals are embedded, has a doleritic structure. The felspathic 

 crystals, with multiple polysynthetic twins according to the albite law, show large 

 extinctions (38° to 41°), which may be compared with those of bytownite ; often two 

 large individuals cross each other. The sections of this mineral are cracked and 

 pervaded with zeolitic matter, which forms an irregular network. This matter, which 



