296 PROP. T. Gr. BONNEY AND MISS C. A. RAISIN ON [May 1 899, 



soft dark chloritic rocks, while a very small projecting knob consists 

 apparently of gabbro. Under the microscope, the rock from the 

 quarry exhibits a finely granular quartz-felspar groundmass, with 

 greenish, probably actinolitic microliths, small lenticular micaceous 

 patches, and occasionally a broken fragment of a large felspar-crystal. 

 This rock is evidently pressure-modified, and probably is derived from 

 a gneiss, the type of structure being somewhat similar to that of the 

 ' mylonitic ' gneiss of Glen Laggan. In the boss farther east, in 

 the same kind of rock, tufts of acicular actinolite have developed, 

 smaller than, but similar to, those which we have described in 

 crushed gabbros. There the tufts have doubtless formed from the 

 pyroxenic constituent ; and the crushed rock before us was probably 

 once a hornblendic gneiss. 



(d) The rock, already mentioned as probably a porphyrite (p. 294), 

 occurs in a small field at Cerig-moelion north-west of the quarry. A 

 craglet of serpentine, partly variolitic (§ II, 3 /, p. 283), rises on the 

 southern slope of the field. About 3 yards from this, a low mound 

 begins, on which are several small bosses of rock (1 to 3 feet across). 

 The nearer of these probably consist of modified gabbro. One of 

 them shows yellowish epidotic patches in a pale green ground, the 

 fine laminae of which are crushed and crumpled, with a slight, rather 

 fibrous lustrous speckling. Under the microscope, the epidote is 

 seen to occur in aggregated granules, or in short, rather thick prisms. 

 The part between the patches consists mainly of matted actinolite, 

 often showing a tendency to a streaky arrangement, and at one 

 place a group or crystal of (?) zoisite occurs. The rock is probably 

 a gabbro modified by pressure. 



Next beyond this (in one boss to the west of the mound, and at 

 the southern corner of one near the centre) is a rock containing 

 abundant tufted actinolite (| inch or more long) in a soft green 

 ground, which includes (as seen under the microscope) flaky chlorite 

 with some granules of either zoisite or a felspar. It is, on the 

 whole, more probable that this rock is not a porphyrite, but has been 

 derived from the gabbro. 



Prom the next few bosses (each about 3 feet broad) several banded 

 specimens were obtained.^ The bands are : — (1 ) A dull pale greenish- 

 grey rock, compact, but finely laminated on the weathered surface : 

 on microscopic examination we see a felsitic groundmass crowded 

 with granular epidote ; (2) a pale greyish-purple rock, similarly 

 compact and laminated, composed of a cryptocrystalline felsitic 

 groundmass, throughout which are scattered very numerous but 

 minute microliths of actinolite and small grains of opacite ; and (3) 

 a similar rock, but darker purple in the hand-specimen, rather finer 

 grained in texture, and more thickly crowded with opacite. Eadial 

 or sheaf-like tufts of a greenish actinolite penetrate all these bands 

 as in the St. Gothard schists, and the crystals are developed some- 



^ The next boss is composed of compact pm*plish rock without actmolitic tufts. 

 A banded porphyrite accordingly seems to extend roughly east and west, north 

 of the gabbro and serpentine. 



