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



in the interior of these sections. Titanite presents whitish grey sections ; these are very 

 sharp rhomboids with traces of a cleavage parallel to two sides of the figure. These 

 cleavage lines should be parallel to the face r (Soo) or I ( co P) ; the two other sides 

 may be, in the first case, P (OP), in the second y (Pco). There are also large sections 

 of magnetite often surrounded by a slight zone of chloritic matter, which also penetrates 

 to the interior of the hornblende. 



A specimen, which may be viewed as related to the preceding rock, is essentially 

 composed of pyroxene and hornblende. It is granular in texture, with rather large 

 grains, and shows biotite as an accessory element. In spite of the analogy with the 

 diorite just described, there is no felspar in the specimen in hand, and it may be 

 viewed as resulting from a more basic concretion such as often occurs in the ancient 

 massive rocks. Hornblende exhibits the same characteristics as in the preceding rock, 

 but is intercalated amongst the minerals ; at other times it is enclosed in augite, and 

 oriented like the latter. The crystals of augite generally show a better preservation of 

 the crystalline form than the hornblende, and have more or less prismatic outlines in 

 the sections, contrasting with the more irregular appearance of the amphibole. This 

 mineral appears to be secondary, resulting from the decomposition of augite. It 

 contains lamellae of biotite, and besides these minerals magnetite is also to be found. 



Some fragments of rock belonging to the series of crystalline schists were collected at 

 Port Sussex. One of these, which to the eye appears covered with ferric oxide, is fine- 

 grained, breaking with a plane fracture pierced with perforations. The ground-mass, 

 when viewed microscopically, is seen to be formed of lamellae of mica — apparently 

 altered biotite— lying in all directions and associated with an amorphous mass. Some 

 sections with indistinct outlines are visible as a microporphyritic mineral ; these 

 sometimes resemble hexagons, and we may have to deal here with altered garnets ; in 

 other cases the sections are prismatic, and they may then be classed as felspar. These 

 sections are often filled with a light greenish secondary material resembling chlorite. 

 Little quartz is to be seen, and finally there are rhombic sections which represent an 

 altered rhombohedric carbonate. 



We may mention amongst the clastic rocks of Port Sussex, a specimen formed of a 

 greenish fine-grained mass, in which no crystalline elements are visible, and enclosing 

 a granitic fragment, of which we shall speak later. The microscope shows this 

 rock to consist of clastic fragments cemented by a ferruginous argillaceous mass. 

 The broken crystals which are to be seen come from the disaggregation of ancient 

 eruptive or schistose rocks. Amongst these minerals, quartz, plagioclase, microcline, 

 orthoclase, and some splinters of almandine garnet are particularly visible. This rock 

 agrees very well with the composition of an arkose, although we have not ascertained 

 the presence of mica. 



