376 REPORT— 1888. 



of similar type, though the general size of the elements was smaller, and 

 the larger fragments ai-e fewer ; but they are characterised, as at Holyhead, 

 by their zircons and possible tourmalines, and other peculiar but uncertain 

 minerals. 



Laminated HocJcs, as defined by the alternation of different materials in 

 bands, are not abundant in Anglesey, but when present they are easily 

 distinguished. They occur only, as far as observed, on one horizon — namely, 

 immediately above the quartzites or equivalent rocks, where the series is 

 complete ; they are thus confined to the Western and Eastern districts. 

 Their structure is admirably seen in the rock from Porth-y-felin, Holy- 

 head (5), in which the laminte are beautifully bent. The rock is now a 

 highly crystalline one, with green-mica distributed abundantly through- 

 out ; but in certain narrow, well-marked lines it becomes much more 

 abundant, and has granules of dust-like epidote mixed with it. Anyone 

 of these bands may be traced continuously from one end of the slide to 

 the other, always keeping its distance from the next, and following every 

 bend in the rock. These bands are of different thicknesses and separated 

 by different intervals ; about twenty of each may be counted in half an 

 inch. The principal part of the rock is quartz, but a few fragments of pla- 

 gioclase are scattered here and there. There are seldom any phenomena 

 indicating pressure after the formation of the rock. These features indi- 

 cate a true laminated original deposit. In another example, from Porth- 

 yr-Corwgl (6), we find the same alternation of more and less micaceous 

 and dusty bands, but here a later pressure has been at work. It has, 

 however, only served to destroy the individuality of the crystals, and to 

 intensify the difference between the bands. These are still continuous 

 and equidistant, and the original fragments of quartz and felspar have 

 not been obliterated. The rocks to the south of Porth-y-defaid (17) are 

 also beautiful examples of this type ; specimens occur which are neither 

 contorted nor pressed, but simply altered by crystallisation, with the 

 derivative fragments relatively large and abundant. It will be remem- 

 bered that not far fi'om here is the place of occurrence of the unaltered 

 slate. The finer-grained rock from near the chapel, Four-mile Bridge 

 (8), described by Professor Bonney as a ' normal schist,' shows the same 

 lamination, but not quite so distinctly, the micaceous bands being 

 broader, and their regularity somewhat interfered with by the introduc- 

 tion of segregation veins of quartz. It is, in fact, a somewhat inter- 

 mediate rock between those of Holyhead and the fine-grained rocks to 

 the north-east. Its lamination is clearly recognised by Professor Bonney, 

 who says the rock ' has a banded structure, probably due to original 

 bedding.' 



In the Eastei-n district the only two rocks of this kind examined have 

 suffered much disturbance, so that their laminated character is much 

 more easily recognised with a hand-magnifier than with a microscope. In 

 that from Tyn-y-mynydd, on Mynydd Llwyddiart (163), there are some 

 bands less disturbed, and these are quite equidistant and continuous. 

 They contain epidote in the lighter bands, and large plates of mica in the 

 darker. Elsewhere they become more contorted. Both here, however, 

 and at Hafodty (145) in the south, the peculiar character of the rock is 

 nowhere lost. 



All the above-named rocks maintain as their present essential character 

 the structure they originally possessed, but there are a large number of 

 others in which metamorphism has gone so far that its results are the 



