70 



or 4 millimetres in length. The colour varies from a brownish- 

 grey to an almost black, pitchy-looking rock. In rocks Nos. 6, 

 10, and 11 decomposition has gone on to a great extent, giv- 

 ing the rocks a greenish appearance, which is probably due to 

 the alteration of the ferromagnesian mineral to serpentine. 

 The locality from which rocks Nos. 6 and 9a were collected fur- 

 nishes a good instance of columnar jointing. Use is made 

 of this character in the quarries near Kingscote and the 

 Bluff, the rock being sufficiently weathered to render it easily 

 workable for road metal. The rocks collected from the Gap 

 Hills also show jointing developed to a less degree, being more 

 or less stained with ferric oxide. As far as hand specimens 

 are concerned, the rocks are very similar to one another. 



Microscopic Characters. 



The rocks have a hypocrystalline porphyritic structure, 

 the phenocrysts being augite, enstatite, and plagioclase, irre- 

 gularly distributed in a base of microlitic piagioclases and 

 augite, with a small proportion of glass in the interstices. Of 

 the phenocrysts, the augite is by far the most abundant con- 

 stituent, possessing only the faintest indication of a light-green 

 colour. They are cracked about a good deal, and somewhat 

 corroded by the base, whilst others again possess a good idio- 

 morphic outline. 



The majority of these phenocrysts are intergrown with 

 the plagioclase of the base in an ophitic manner (plate xx., 

 fig. 1). Twinning is not an uncommon feature, and more 

 rarely an hour-glass structure is developed. In the fresh 

 varieties of the rock only a slight decomposition to a brown 

 stain is noticeable in the cracks. 



The mineral next in abundance is the enstatite, being 

 corroded to a greater degree than the augite, and including a 

 few tabula? of felspar. The phenocrysts of the enstatite are, 

 in a great many instances, partially or wholly enwrapped by 

 the augite. The enstatite has evidently crystallized out first 

 of the ferromagnesian constituents, and the augite has used 

 it as a nucleus to crystallize on later. In two or three in- 

 stances cited the corroded enstatite has a distinct resorption 

 rim developed, and the augite has crystallized around the 

 whole. The difference between the enstatite and augite in 

 such a combination is particularly marked when examined 

 between crossed nicols : the former, when in the position of 

 extinction, shows the coloured envelope of augite which is 

 extinguished at only 45° to this position. 



The ferromagnesian constituents as a whole are nearly 

 colourless, and this may account for the relatively low per- 



