REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



119 



brilliant colours of polarisation. A greenish secondary product covering a considerable 

 part of the preparation appears, and sometimes assumes a vermicular form very like 

 that of helminth. 



Two specimens of basalt were collected on the summit of Table Mountain. One of 

 these was taken from a bed the rocks of which showed columnar structure. It is a 

 very compact bluish black basalt, with a plane fracture, and contains large inclusions 

 of olivine. Under the microscope the rock is very fine-grained, and in the ground-mass 

 greenish brown augite crystalloids predominate, embedded in plagioclastic lamellae. 

 Fragments of olivine detached from a large inclusion of a peridotic rock are also to 

 be seen. Rather large patches, composed 

 exclusively of augite grains, are sometimes 

 to be observed. The bottle-green nodules of 

 olivine, enclosed in this basalt, are formed 

 by an aggregation of minerals which corres- 

 ponds to lherzolite (see fig. 20). Olivine 

 forms the principal mass of this inclusion, 

 its grains appearing irregular, colourless, 

 and split up without a trace of definite 

 cleavage (a). A lamellar rhombic pyroxene 

 is associated with this mineral; its colour 

 is light green, and it is probably en- 

 statite (b). Finally, transparent brown 

 isotropic sections of picotite and greenish 

 augite (c) are embedded without crystalline 

 outlines amongst the minerals already 

 mentioned, moulding themselves upon them 



Another preparation from one of the peridotic nodules of the Table Mountain basalt 

 shows a slightly different composition. In this case the rock appears to be formed only 

 of olivine, the aggregated grains of which have experienced a slight serpentinisation 

 along the cracks. 



The second specimen from the upper part of the mountain is, like that briefly 

 described above, an ordinary black, compact, fine-grained basalt, in which the eye can 

 detect nothing but grains of olivine. The ground-mass is formed of small plagioclastic 

 lamellae, not much lengthened, and of brownish granules of augite with which magnetite 

 is associated. Large fragments of olivine without crystalline outlines give the rock a 

 microporphyritic structure. One cannot help recognising these fragments of olivine as 

 foreign inclusions, and similarly a like origin must be admitted for the large sections of 

 chromite which the rock contains. These may be as much as two to three millimetres in 

 diameter ; they are very irregular in outline, and often surrounded by a zone of magnetite. 



FlO. 20.— Basalt of Table Mountain. 



Microscopic section of an inclusion in this rock. The inclusion 

 is formed {«) of olivine in cracked, colourless, irregular 

 grains ; (6) rhombic pyroxene, lamellated, and light green 

 in colour ; (c) greenish grains of augite. The inclusion 

 also contains brownish sections of chromite or picotite, 

 which are not figured. 5 ' 5 crossed nicols. 



