Tol. 55.] SEEPENTrSTE AND ASSOCIATED E0CK8 IN ANGLESEY. 291 



Enstatite-rock. — This apparently forms a dyke (12 to 18 

 inches wide) cutting a low boss of serpentine near the northern 

 shore of the large inlet south of Penrhyn-Fadog. Crystals of 

 enstatite are conspicuous on an exposed surface. These are about 

 ■1 inch long, markedly idiomorphic (bounded by faces 010 and 110), 

 having fresh-looking cleavage-planes (with brassy lustre) crossed by 

 two sets of cleavage-lines at right angles to each other. The 

 bright surfaces are surrounded by a dull, pale green substance, 

 which indicates cross-fractured crystals. Under the microscope the 

 rock is seen to consist mainly of enstatite partly altered to a faintly 

 pleochroic ^ mineral and a steatite-like aggregate. In one or two 

 cases, crystals of the one character are apparently enclosed in 

 crystals of the other. A slice obtained from a rather crushed band 

 shows a rudely granular structure, and is traversed by brown-stained 

 cracks which otten, but not always, follow the outline of the grains. 

 A few small grains occur, which are perhaps perofskite. 



In the islet at the mouth of the inlet a rather more extensive 

 outcrop of a similar rock interrupts the serpentine. It forms low 

 reefs along the western beach of the islet, rising and broadening at 

 the southern end into a considerable crag (becoming schistose in 

 parts), and it just appears about the middle of the islet at the foot of 

 the eastern cliff adjoining the serpentine. The microscopic characters 

 are very similar to those of the rock described in the preceding 

 paragraph, but one slice contains a few grains of augite, and in 

 another an actinolitic growth occurs which is described on p. 291. 

 Very near the islet, in a low reef at the entrance of Penrhyn-Fadog 

 Inlet, is yet another enstatite-rock, probably also intrusive, but no 

 junction is exposed. This, however, contains a small amount of 

 intercrystallized diallage. Similarly, a small intruded mass in a 

 boss at Cerig-moelion is a rock now composed of serpentine and 

 diallage, but microscopic examination shows the serpentine to have 

 been formed from enstatite. 



Again, an enstatite-rock forms the point of a skerry projecting 

 from thousand, near the boss first described. It is pale greenish or 

 whitish, at places almost fibrous, and in the field much resembles a 

 slightly crushed variety of the diallage-rock. Under the microscope, 

 the slice shows a curious mottling of two constituents : one is 

 ferriferous, apparently an altered enstatite ; the other is a clear 

 serpentinous mineral. Some of the latter replaces a variety of 

 enstatite ; the only clue to the origin of the remainder is that it 

 does not show the characteristic structure of serpentine formed from 

 olivine. The slice is partly crossed by a vein-like band of a fibrous 

 mineral with fairly bright polarization-tints and straight extinction : 

 this is probably a variety of serpentine. 



Thus there is proof of the intrusion into serpentine of (1) a 

 diallage-rock, (2) an enstatite-rock, and (3) rocks formed of the two 

 •minerals combined in varjdng proportions : that is, of pyroxenites, 

 using the term in a rather extended sense. Where, however, the 

 enstatite has become serpentinized, especially if the rock forms 

 ^ Markedly pleochroic in a thicker slice. 



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