BY W, N. BENSON. 597 



size. If they be a portion of the same rock-mass as the Tiii- 

 tinhull rocks, as seems very probable, the conditions of consolida- 

 tion must have been different. The porphyritic spilites on East Gap 

 Hill are very coarsely granular, where they are invaded by the 

 dolerite near the summit of the hill, and diminish in grainsize, and 

 increase in vesicularity as they are traced to the south. The more 

 coarsely granular types contain large phenocrysts in a subvarioli- 

 tic groundmass (1032, 1033, 1168). The phenocrysts are idiomor- 

 phie plagioclases, but are moulded on augite, when the two minerals 

 come into contact. They may be as much as 8 mm. in length. It 

 is difficult to determine their composition, as the crystals are rather 

 dusty, and full of partially chloritised zonary inclusions of the 

 groundmass, and of epidote. They seem to be albite, however. 

 They are not zoned, and have been more or less replaced by a 

 mosaic of minutely granular quartz, which may, in some cases, 

 leave only the outer rim of the felspar intact. Phenocrysts of 

 augite also occur, either singly or in aggregates. They are rather 

 decomposed, and may include large grains of magnetite. Some 

 octahedra of magnetite also form phenocrysts. The groundmass 

 consists of lathy oligoclase with forked microlitic extensions, skele- 

 tal forms of ilmenite, as well as definite grains, and small grains of 

 titanomorphite. The pyroxene, which was originally in the form 

 of small prisms, is now completely changed to chlorite and epidote. 

 Irregular patches of secondary quartz-mosaic also occur in the 

 base. The vesicles are numerous, and dit¥er in their content of 

 minerals. The following types have been noted: — 



(fl) Outer margin, epidote; central portion, orthoclase. 



f'^jOuter margin, epidote and chlorite; inner portion, quartz. 



(c) Outer margin, epidote and chlorite; inner portion, quartz- 

 mosaic. 



(6?) Outer margin, epidote; central portion, chlorite. 



(e) Outer margin, epidote; inner portion, chlorite; centre, epi- 

 dote. 



(/) Epidote, chlorite and axinite, irregularly arranged. 



(g) Epidote, chlorite, felspar and axinite. 



