392 REPOBT— 1888. 



have a layer of sericite clinging closely to their surface. At the same 

 time they are clearer, the specks in them are not scattered irregularly, or 

 absent from an external band, but lie in different planes produced by 

 cracks which go across from side to side and intersect each other ; many 

 of them are also crowded with fine crystalline needles. These rounded 

 grains are not at all like any of the fragments in the ordinary quartzites, 

 nor like any authigenetic element in the schists, more especially in the 

 particular of the fine needles — these point to a source such as that which 

 produces granitic rocks. In any case these rounded derivatives are 

 sharply marked off by all their characters from the remainder of the 

 rock, which consists of polygonal authigenetic elements of almost com- 

 plete purity, with the exception of a little sericite. The rock thus con- 

 trasts strongly with any of the bedded quartzites, notwithstanding that 

 it is quite small in bulk, and surrounded by slaty rocks, and the peculiar 

 roundness of the derivatives presumes a long attrition. Have these been 

 worn by the very water from which the matrix has derived its silica ? and 

 has the attrition taken place, not in streams on the surface, but in cir- 

 culating currents underground ? 



From this we may pass to the knob of quartz at Pen-y-Parc, Beau- 

 maris (203), described by Professor Bonney. In this the derivatives are 

 very numerous, and not all of quartz ; yet is the general character of the 

 rock the same, and it has the characteristic mode of occurrence, and it 

 is impossible to imagine it brought into its present place by folding. In 

 fact, though the neighbouring rocks have been shown to be crowded 

 with mylonitic lines, no spectral polarisation affects the quartz. On the 

 contrary, one of the derivatives shows microspectral polarisation de- 

 veloped before its deposit, and not continued into the surrounding matrix. 

 Notwithstanding, therefore, its great similarity, in many respects, to an 

 ordinary quartzite, we may conclude that the circumstances of its origin 

 were different, and the general method of production the same as in the 

 other quartz knobs. 



There are many other similar quartz knobs in the island, which have 

 not been examined, but the great mass at Llangefni (97), marked ' green- 

 stone ' on the Survey map, is peculiar. It is much larger than any other, 

 and from a distance looks to have almost a bedded appearance. There 

 are a great many derivative fragments in it, but in the character of these 

 and of the matrix it much more closely resembles those above described 

 than any of the ordinary quartzites. 



Green Eocks. — Besides the limestones and quartz rocks, there occur 

 occasionally, in similar isolated masses which often weather into knobs, 

 certain green rocks which show no structure on the large scale but look 

 minutely granular. These are found to be composed of minute crystals 

 of definite minerals, which seem to have segregated in such separate 

 masses. Some, as at Borthwen, near Roscolyn (68), are composed of 

 epidote, whose crystals are separated by lines of black dust, and with 

 shrinkage areas filled with quartz. Others, as at Perth Delise (14), which 

 are more fibrous-looking, are composed of a greenish transparent mineral 

 arranged in tufts, which polarise with a grey tint like serpentine, of which 

 they are probably a variety. In another, south of Beaumaris (192), the 

 nature of the principal dusty mineral without any form is obscure, and 

 may be decomposed epidote ; the green colour is produced by the ground- 

 mass of chlorite. These masses are mostly confined to the most altered 

 rocks, and seem, like the others of this group, to have a chemical aqueous 

 origin. 



