ON THE OLDER BOCKS OF ANGLESEY. 369 



the rocks of igneous origin. In these cases it cannot be considered true 

 twinning which produces the bands, but a reconstruction under pressure 

 of a single mineral. There are, however, in numerous rocks small angular 

 fragments, in which the lines of twinning have the utmost regularity and 

 definition — e.g., at Llaneilian (223). These are probably true plagioclase. 

 As far as observed they invariably are derivative, being angular and 

 isolated, and often lie transverse to the general direction of foliation. 



Mica. — The most characteristic species of this family observed in the 

 sedimentary rocks is muscovite. It is colourless for all rays, and polarises 

 vividly. It is commonly of small size, but is best developed where the other 

 elements are large, as in the gneisses and mica schists of the Eastern and 

 Central districts. It also occurs as large fragments in the midst of a 

 non-micaceous matrix in the slates of Roscolyn {^Q^), and the quartzite 

 of Porth-yr-Ogof, Holyhead (4). Inclusions are rare, but in a rock full of 

 mica, on the Llanerchymedd Railway (93), the larger elements are full of 

 other crystals, thus resembling a speckled felspar. 



Chlorite. — It appears that two distinct species have been referred to 

 under this name, the only common characters being greenness and 

 transparency. The true chlorite is more or less amorphous, and occurs 

 in large sheets as well as more fibrous masses. It often forms the whole 

 groundmass of the rock, as at Bryn Minceg, near Llandegfan (189). It 

 also occurs as beautiful vermicular aggregates amongst the large vein 

 substance, as in the mica schist of Bodowyr (146) and Gaerwen (160) ; 

 and occasionally it crystallises transversely in a lenticle, as at Roscolyn. 

 (Q7). In these cases it has very little polarising power, remaining nearly- 

 dark between crossed Nicols, or giving bluish and purplish tints. There 

 is not much of such chlorite in the Western district, but it is more- 

 abundant in the Eastern, where also epidote more commonly occurs, after- 

 ■which it is often found as a decomposition product, as in the rock at 

 Hafodty (145). 



Oreen-viica. — This is the so-called chlorite of the Western district, 

 being the essential ingredient in the ' chloritic ' schists. It occurs in 

 Jong thin crystalline flakes, with parallel sides but imperfect termina- 

 tions, and may be either isolated or aggregated in groups. It is trans- 

 parent and greenish in tint, and extinguishes parallel to the trace of 

 cleavage. The polarisation colours are either brown or fairly brilliant 

 tints of red and green. It is often surrounded by the true chlorite as . 

 by a decomposition product. 



Sericite. — This term is applied to the colourless strings and broader 

 patches in which no individuals can be distinguished, but which polar- 

 ises in fairly brilliant colours between crossed Nicols. It occupies the- 

 cracks and minute interstices between the other elements, and is espe- 

 cially characteristic of rocks which have been partially disintegrated by- 

 pressure. 



Epidote. — It is generally supposed that this species is a decomposition 

 product from felspar. In the sedimentary rocks of Anglesey there is no 

 special evidence that this is its direct source. It is here always highly 

 fractured and more or less dusty, and occurs in isodiametric patches 

 ■without crystalline form. It shows high polarisation tints in the centre, 

 "which change rapidly towards the edges, owing to the thinning off of the 

 grains. It gives rise by decomposition to dust of a characteristically 

 crystalline aspect, by which it may be recognised even when too small 

 or decayed to show colour between the Nicols. It is most abundant in 



1888. B B 



