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



rather small angle, which leads one to believe that the plagioclase is akin to oligoclase 

 or andesine. Some individuals are found extinguishing parallel to the lengthened 

 sides, and showing no plagioclastic lamellae, which would seem to indicate that they are 

 orthoclase. It is difficult to decide this question, but the hypothesis is not without 

 some basis, since the rock presents the association of quartz and felspar, which is known 

 as micropegmatite. Now it is well known that no plagioclase intergrows in this 

 manner with quartz. We lay no stress on the secondary minerals ; the epidote appears 

 as in the diorites already described, quartz of secondary formation is abundant, and 

 also lamellae of chlorite united and entangled with epidote. These minerals either 

 penetrate the entire ground-mass, or have crystallised in microscopic geodes and fissures. 

 The specimen examined is not homogeneous, and everything points to the conclusion 

 that it is a volcanic tufa of ancient tjTpe, but decomposition has proceeded so far that no 

 definite opinion can be arrived at on this point. The rock just noticed may be 

 described as related to the diorite type, to which it shows special affinities in the small 

 angle of extinction of the felspar. Another one now to be described departs altogether 

 from this type. It is fine-grained and crystalline, with numerous small crystals of 

 plagioclase and augite, and greenish black brilliant scales of a chloritic mineral. It 

 contains a black mass which seems to have been enclosed ; the fracture is almost 

 plane. The augite crystals and the very high angle of extinction of the plagioclase 

 distinguish this rock from the preceding. Plagioclase plays an important part in it, 

 appearing in the form of large crystals or aggregations, and being twinned according to 

 the albite and pericline laws. In the sections in which hemitropic striae appear (those 

 following the albite law crossing those of pericline at right angles), the symmetrical 

 angle of extinction for the polysynthetic lamellae may rise above 30°, which seems to 

 indicate that this felspar is not far removed from bytownite or anorthite. Pyroxene 

 appears in the form of rather large rounded crystals often twinned polysynthetically 

 according to the ordinary law. Its colour is not dark, sometimes indeed a very 

 pale yellow tint. This mineral has undergone mechanical changes which have given its 

 sections a fragmentary appearance ; they are decomposed on the surface, and calcite 

 has crystallised along the edges. The augite contains cavities that may have been 

 originally vitreous, but they have been modified by decomposition, which has also altered 

 the base, probably vitreous at its origin, and transformed it in great part into secondary 

 quartz and matter resembling chlorite. Although it is extremely difficult to give a 

 decided opinion on the nomenclature to apply to rocks collected in isolated fragments 

 and which have undergone great alteration, still, by taking account of the texture, the 

 mineral association, and the special characters of the augite and plagioclase, one may 

 venture to class the specimen under consideration with the diabases. 



A greenish pebble with irregular fracture, sprinkled with large, more or less circular, 

 patches of calcite, contains only one macroscopical mineral, greenish in colour, probably 



