146 MESSRS. C. I. GARDINER & S. H. REYNOLDS [Feb. 1 898, 



east of the Castle is of the same type as the mass near Trinity Well, 

 and seems to be intruded along the junction of the andesites and 

 slates. 



The two exposures of the porphyrite, one of which forms the 

 conical hill at the south-western end of Pilot's Hill, and the other 

 Plint Eock and part of Bell Rock, resemble one another exactly. 

 The groundmass is more prominent than in most of the coarse 

 rocks, and of a grey-green colour, while the felspars are thinner 

 than the usual porphyritic felspars ; they are about 7 mm. long, 

 and show a markedly parallel arrangement. On the northern 

 slopes of the conical hill, mentioned above as forming the south- 

 western end of Pilot's Hill, the coarse rock is extremel}'' amygda- 

 loidal. The amygdules, formed of calcite, weather out into round 

 masses, sometimes 17 mm. long, and occasionally form by far the 

 largest portion of the rock. This very amygdaloidal porphyrite is 

 also seen on the northern slopes of Flint Rock. 



Another peculiarity of the coarse rock of these two exposures is 

 the presence of a large number of included fragments. These are 

 of very irregular shapes, angular in outline, and generally blue or 

 purple in colour ; they are consequently very obvious in the faces 

 of rock exposed. They vary much in size, the largest one seen 

 being about 12 inches long ; they all have a somewhat striped 

 appearance, break with a fairly perfect conchoidal fracture, and 

 appear somewhat porcelainized. In fact, they look like baked 

 ashes, and under the microscope they are seen to be fine ashes 

 which have undergone considerable alteration. 



The base of the mass which forms part of Pilot's Hill was seen in 

 the cliff just above sea-level; it was slightly amygdaloidal, enclosed 

 blocks of andesite, and rested on an andesitic breccia, beneath 

 which came a red amygdaloidal andesite. 



It appears, therefore, from these two exposures of the coarse rock 

 that we have evidence of the actual outflow of the porphyrite, for 

 we can see the brecciated surface of the underlying andesite in the 

 sea-cliffs, the slightly amygdaloidal base of the porphyrite (which 

 caught up and enclosed portions of the rock beneath it), and its 

 extremely amygdaloidal nature along the northern slopes of the 

 two exposures, while the closer grain and appearance of the rock 

 also mark it out from the other coarse porphyrites of the island. 



The other exposures of the rock are small in extent, though 

 numerous, and show the porphyrite penetrating the finer andesite in 

 dykes of various lengths. Here and there they accompanj^ exposures 

 of breccia, which seems to point to explosive action having caused, 

 or having assisted in causing, the opening in the andesites, which 

 was afterwards filled with the coarser rock. 



Two sections from the coarse porphyrite show a micropoikilitic 

 structure in the groundmass. One of these is from the edge of 

 the Trinity "Well mass, and the other from a small exposure of the 

 rock south-west of Heath Hill. In one of these the freshness of 

 the porphyritic felspars, as was the case with the andesites showing 

 micropoikilitic structure, is remarkable. 



