294 PKOF. T. G. BONNET AND MISS C. A. KAISIN ON [May 1 899, 



(3) Porphyrite (?). 



A rather slaty -looking purplish rock, which somewhat resembles 

 a crushed compact porphyrite, occurs in several places among the 

 schists or schistose rocks, but the rock could not be identified 

 precisely without careful examination of each specimen. At one 

 locality, however, north-east of Cerig-moelion, whence sections have 

 been prepared, it appears to be a porphyrite. This, as described 

 on p. 296, contains some peculiar tufts of actinolite. 



(4) Greenstones. 



Greenstone-dykes cutting the gabbro are well seen in several bosses 

 south of Llyn Dinam and west of Llyn Penrhyn. On one surface 

 four almost parallel d3^kes can be traced : the narrowest (2 inches^ 

 wide) compact and dull green, the broadest (about 5 feet across) 

 apparently a basalt or very fine-grained dolerite (not in good 

 preservation). 



Junction-specimens from another part, where the dyke is branched, 

 have been sliced. These show the gabbro with a rather uneven 

 edge, and a compact opaque magma-basalt, crowded with microliths, 

 and containing microporphyritic crystals, often arranged in stellate 

 or other clusters : these being (1) lath-shaped plagioclase-felspar, 

 sometimes reconstituted, with an aggregate structure ; (2) augite ; 

 and (3) a green or brown silicate, associated with opacite, probably 

 serpentinized enstatite. 



A massive dolerite-dyke can be recognized at intervals over a 

 considerable distance, as shown on the Geological Survey map. It 

 cuts the serpentine of Holyhead Island and extends near the south- 

 western boundary to the cottage by Plas-coch. It is doubtless con- 

 nected with the dyke of coarse dolerite near Holyhead, to which it 

 bears a general resemblance. It is perhaps rather finer-grained, 

 but we have not had it sliced, as this seemed unnecessary. 



V. ACTINOLITE-TUFTS IN PoCKS NEAR JUNCTIONS. 



Slices cut for microscopic examination from several specimens, 

 which are evidently near a junction of two rocks, show tufts of 

 acicular crystals often diverging in 02)posite directions from a centre. 

 Where best developed, the prisms are long and narrow (say about- 

 •08 by "002 inch) with even sides, though they are somewhat 

 jagged at their ends, and are not infrequently curved, forming- 

 sheaf-like groups (PI. XXIII, fig. 6). The mineral is generally a. 

 clear, pale green, with the extinction and occasionally even the- 

 lattice-cleavage of hornblende. The tufts traverse the rock in ali- 

 directions, and sometimes penetrate into veins or, where the rock 

 is banded, into adjacent layers : the prisms being often interrupted, 

 by inclusions of crystalline grains. Hence we deem them of 



