394 EEPORT— 1888. 



rocks which for other reasons are knowa to have been subjected to snch 

 pressure, as in the neighbourhood of Llanfaelog and Gwalchmai. Some 

 felspars also show a broad obscure banding, independent of all the above, 

 which probably represents an imperfect zonal structure. 



Mica is not abundant in this group of rocks, except in isolated exam- 

 pies, and in the more basic group it is generally entirely absent. Perhaps 

 muscovite is the most generally diffused species ; but whereas ordinary 

 biotite is entirely absent from the sedimentary series, it is here nearly 

 as abundant as any other, to judge by the darker colour and dichroism. 

 It is especially characteristic of the cleanest granites, but is most, abun- 

 dant where the rock is foliated at the margins of the granite areas, as at 

 Tafarn-y-botel (123) and Ty Newydd ('2i5), and in the district north- 

 east of Parys Mountain (251), 



Hornblende may be considered the characteristic mineral of the rocks 

 of igneous habit, inasmuch as it never occurs in the sedimentary rocks of 

 Anglesey, either as a derivative or authigenetic. A large number, how- 

 ever, of these igneous rocks contain it as an essential ingredient, always 

 in the common green form. It is entirely absent from the rocks of the 

 l^orthern district, but occurs in the district east of Parys Mountain. In 

 the Western district it is found only at Pen-bryn-yr-Eglwys, and at Llyn 

 TrefwU, in both cases in association with granitic rocks. The principal 

 localities, however, are in the Central and Eastern districts, where it 

 sometimes forms the greater part of the rock, as near Holland Arms (181). 



Glaucophane. — The occurrence of this mineral in the Anglesey schists 

 has recently been recorded by the writer (' Geol. Mag.' N.S. Dec. iii. vol. v. 

 p. 125). It is there stated to occur in long narrow prisms with incom- 

 plete terminations, which have a rhombic cross section with an angle of 

 about 124°, sometimes modified by the clinopinakoid. The extinction, 

 as far as can be determined, makes an angle of about 15° with the length 

 of the prism, which is somewhat greater than the characteristic amount, 

 and approaches the angle of hornblende. Ilence Italians would call the 

 mineral 'gastaldite.' The characteristic blue colour is seen when the 

 length of the prism coincides with the short axis of the polariser, and it 

 has a violet tint when placed in the perpendicular direction. In the 

 rhombic section it has a slightly yellowish tint. The crystals are usually 

 interfelted, so as to have no definite boundaries, but they are occasionally 

 seen floating separately in the layers of quartz which sometimes peneti-ate 

 the rock. This mineral has only been hitherto found in one very schistose 

 group of rocks in the Eastern district, and even there it passes locally 

 into hornblende and decomposes into chlorite. It is best seen in the rock 

 near Anglesey monument (168, 169). For a figure of this rock see 

 Teall's ' British Petrography,' plate 37. 



Epidote is perhaps the most widely distributed of all the minerals in 

 these rocks, though from its small comparative amount in each the 

 aggregate is not so great as of others. It usually occurs in isodiametric 

 elements, often with crystal outlines, and occasionally lath-shaped and 

 radiating. It also occurs in long pulled-out bands in rocks which have 

 been dynamically metamorphosed. It is most abundant in the gabbro 

 schists of the Western district, in the diorites of the Central district, and 

 in the hornblende and glaucophane schists of the Eastern district. In 

 many cases there is no direct evidence of its being a secondary product ; 

 and in one case, that of a spheroidal rock, near Amlwch (227), it appears 

 to have developed out of felspathic groundmass, without actual felspar 

 preceding it. 



