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THE VOYAGE OF H.M.S. CHALLENGER. 



olivine, augite, and rather large grains of magnetite, embedded in a ground-mass of 

 minute plagioclase and augite microliths and a vitreous base. The plagioclase sections 

 have very sharp outlines, and can thus be determined with a precision rarely attained in 

 the study of rocks of this nature. It is at once apparent that the plagioclase crystals 

 occur habitually in groups of several, united more or less regularly, often parallel to M, 

 and presenting all the peculiarities of certain macroscopic crystals of albite, those of 

 Schmirn, for example, and of some crystals of labradorite. In many cases the sections 

 of plagioclase have the form of a nearly rectangular parallelogram, with polysynthetic 

 twins and symmetrical extinction ; these sections are thus in the zone P :h, and, since 



From a sketch by Mr. Buchanan, representing the mountainous promontory forming the north-western end of 

 the island. The top of the mountain was enveloped in cloud, below which the greater part of its sides were 

 covered by a glacier descending to the edge of the precipitous rock cliffs, over which the ice-masses fell thundering. 

 The sketch was taken from the shoulder of a red conical hill, against which the ice, descending from the main 

 mountain of the island to the sea, splits and passes on both sides of it. 



they are approximately at right angles, we may conclude that the crystals are tabular, 

 and terminated only by the faces of this zone. Sometimes, but more rarely, plagioclase 

 sections are observed bounded on one side by angles of about 90°, and on the other by 

 more or less obtuse angles formed by the trace of two edges, which may correspond 

 to T and I, and are in general but slightly developed. The sections parallel to M 

 enable the form of the crystals to be ascertained, and the optical properties determined. 

 Sections parallel to this face appear as sharply outlined disymmetrical hexagons, which 



