^'^^- 55-] SEKP£>^TINE AND ASSOCIATED ROCKS IN AlsGLESEY. 279 



(b) The Coast west of the Strait. 



On the coast, south of the muddy shore extending from Pour- 

 mile-Bridge, are small reefs of schist or of schistose rocks. The low 

 cliffs on the north of the first small inlet iuclude variolitic rock (pale 

 green, and associated with a carbonate), a much-crushed, probably 

 diallage-rock, gabbro, and serpentine. A series of ' schists ' below 

 the gabbro-crag have suffered great crushing, which probably acted 

 along a thrust-plane. 



Southward, at intervals, occurs serpentine with interspersed 

 varieties of enstatite-serpentine and diallage-serpentine ; and ensta- 

 tite-rock is found in the low point to the souths in the oull} ing islet, 

 and as a small dyke along the northern share of the next large 

 inlet. Here a diullage-rock is still better exposed as a dyke with 

 branching veins. Earther along this shore (south of Rhyd-bont) is 

 a serpentine with very large crystals of lustre-mottled diallage and 

 enstatite (see pp. 260, 285). 



South of the inlet, serpentine is soon followed by gabbro, which 

 forms consjjicuous masses. On the shore below Dinas-bach a dyke 

 of gabbro,^ running roughly from west to east, cuts the ser- 

 })entine. The outlying rook in the sand is a green ' schist,' which 

 may be partly, if not wholly, a modified gabbro or diabase. Hock, 

 probably similar, occurs at intervals in the peninsula to the eastward. 

 An ophicalcite is found in the field near Dinas-bach, while serpentine, 

 tjabbro, and associated rocks occur near to and east of Fado2;. 



(c) Serpentine. 



In the interior of the district, bosses and crags of serpentine 

 exhibit variations chiefly due to accessory minerals. An example 

 with lustre-mottled crystals, mainly of diallage, occurs south of 

 Cerig-moelion Farm. 



The serpentine includes many variolitic patches, which occur 

 rather towards the northern boundary, as if they might continue 

 the line of the Graig-fawr outcrops. A variolitic structure is some- 

 times suggested rather than clearly indicated (as in the fields around 

 and south of Ehyd-bont), but the patches of well-developed varioles 

 are small, the largest being not more than 10 to 20 feet across. 



(d) Chief Gabbro -Masses. 



Large masses of gabbro are found at several places, generally 

 forming steep-sided plateau-like rocks. Thus one smaller boss 

 occurs north of the head of the large inlet, south of Penrhyn-Fadog ; 

 while south-east of this, close to the mud, rises a cliff of gabbro, 

 almost joining the serpentine which is quarried near at hand." 

 Farther down the inlet is the Graig-Dinas mass (one of the finest 

 in this district), and other bosses are close by, including m iny in 

 two fields north-west of Gareg-wen covert. These are roughly oval 

 in shape, and the schistose planes or lamina3 are generally parallel 



^ Apparently enstatite-gabbro ; see p. 293. 

 - This juaction is described on p. 295. 



