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From what has been said of the nature of these junctions, it may be a question, whether the quartz rock and chlorite schist are not members of the same formation. For, whilst the junctions at each of the Western boundaries of the former bespeak a nonconformity, those on the Eastern sides present a certain degree of intermixture. Supposing the quartz rock to have been once horizontal, and the chlorite schist reposing upon it, perhaps their present appearance may be accounted for upon the principle of an upheaving force acting obliquely from the East. Around Holyhead the chlorite schist is greenish grey, and composed of nearly equal proportions of quartz and chlorite, both highly crystalline, and finely granular. Sometimes it possesses but an imperfectly fissile texture (12.), though in general this is sufficiently apparent (14.), and the quartz and chlorite predominate in alternate layers (15.). The scales of chlorite often form one continuous shining plate on the plane of cleavage (13.). There are still finer grained varieties (16.), where the intermixture of the quartz and chlorite is very uniform. These form an intermediate passage between the crystalline chlorite schist, and bright green silky clay slate. Quartz sometimes predominates considerably (17.). Contortions of the most complicated nature are exhibited in many portions of this series. A large block will often present laminae waving in regular vandykes, or intermixing in a most confused manner (18.). The finer grained varieties appear to predominate through the remainder of the chlorite schist situate towards the Western side of the Island. The general character of the same rock on the Eastern side is slightly different, though the composition is similar. Both the grains of quartz and the scales of chlorite are larger, and the colour of a darker green. Lenticular plates of
pure crystalline quartz, lie in the direction of the layers of chlorite, which pass regularly round them (19.). Strings of crystallized quartz and scales of chlorite intermix in irregular layers (21.), which appear to arise from the complicated contortions of thin laminae. Sometimes the rock is nearly homogeneous, the chlorite being dispersed through the quartz (23.). Some hard varieties occur in the neighbourhood of a rock
5, upon which there is erected a pillar to the Marquis of Anglesea,
near Plas-Newydd. The basis is fine grained quartz, and dark grey chlorite (24) closely united, and through the mass are disseminated small crystalline patches of light yellow epidote, and others of reddish mica. The epidote, in some cases, forms a considerable ingredient (25.), and the scales of chlorite are replaced by dark green spiculae, probably hornblende. Mica slate occurs in the Eastern chloritic district, which lies to the S.E. of a line drawn from Pentraeth to Newborough. This is, however, always contaminated with some portion of chlorite, which may be detected by the earthy smell, even of the purest specimens. The hill West of Penmynydd, about the centre of this district, on the main road from Bangor to Holyhead, and the Llydiart mountain, at its N.W. termination, afford the most crystalline and genuine examples of this rock (26–29.). The quartz does not always present a distinctly granular appearance; but rather constitutes a nearly uniform mass, through which the scales of mica or chlorite are dispersed. It is not always easy to determine whether mica or chlorite forms the second ingredient. Sometimes both are present, and sometimes the quartz is tinged green by an intimate mixture with the chlorite, and scales of white, or light green, mica are superadded. On the shore, between the Menai bridge and Plas-Newydd, mica enters in a conspicuous manner as an ingredient of the schist, and forms large thin plates which are irregularly inter. mixed with an impure chlorite schist (30–32.). Near Cadnant the scales appear to be intermediate between mica and chlorite, and coat over the whole surface of cleavage (33.). Other varieties afford an impure mixture intermediate between clay slate and mica slate, where the scales of mica are sufficiently distinct, but the basis no longer retains a crystalline character (35–37.). North-east of Bodwrog, on the confines of the granitic district, is a variety composed of crystalline white quartz in layers, coated with a talcose variety of mica of a light straw colour (38, 39.). Mica slate occurs to the South of this spot, along the Eastern boundary of the granite, where it does not present a granular aggregate of quartz and mica, but forms a highly crystalline mass of quartz, to which a laminar tendency is given by thin layers of mica, sufficiently distinct on the surfaces of the laminae, and but faintly marked by lines on the fracture perpendicular to them. Where the rock is weathered smooth, the quartz glistens in the same manner as a polished surface of crystalline marble (40.).
A similar variety is found on the East of Tre-Sgawen (41.), c. s.
situate towards the North-eastern termination of this district.
The passage from the crystalline varieties of chlorite schist to the more earthy kinds (43–49), and finally to clay slate (50–64.), is very gradual. The yellow epidote, before-mentioned, also assumes a compact appearance, and runs in irregular strings among the schist (43,44.).
Some specimens of the clay slate present a silky lustre (50), and fibrous appearance. Patches of deep red occur intermixed with the green.
These varieties of clay slate predominate to the North of a line drawn from Llaneilian to Llanfechell. On the coast, from
C. 1. Llanrhyddlyd to the nearest point to Holyhead Island, they
intermix with the crystalline varieties of chlorite schist, in the most confused manner. A boundary may once have existed between them, for the transition from one to the other is frequently abrupt, and resembles a series of patches of clay slate scattered over a ground of chlorite schist, sometimes presenting a distinctly laminar tendency, at others not a trace. The clay slate often assumes a hard jaspideous aspect (59–61). Some earthy varieties, without a trace of fissile texture, appear to consist of an irregular mixture of chlorite and epidote, with patches of quartz, and carbonate of lime (65–69.), intimately
united. These prevail on the coast from Beaumaris to Cadnant,
in the neighbourhood of Llangaffo, and also between Trefdraeth
. and Aberfraw.
Rugged, projecting masses of schist, with laminae generally much contorted, are scattered throughout the tract on which
. Holyhead is situate. The average bearing of the laminae is de
cidedly towards the N.E. and S.W., and their dip in general to the N.W. In the Northern district round Llanfechell, the appearance is similar, but the laminae are not so much contorted, and they dip more to the North. Towards Amlwch they nearly regain their former position, and between this place and Llaneilian, the contortions are very complicated.
Where the rock had been vertically and smoothly cut, in a recent excavation opposite to the pier at Holyhead, there was a decided appearance of broad strata, undulating in a manner similar to those of the quartz rock, and also a laminar structure parallel to the seams which marked the stratification.
A similar circumstance occurs in the high ground to the C. 2. North of Llanbabo; the surface is modified by the undulating nature of the strata, which rest upon each other conformably, and are from two to three feet thick. The schist is very flinty, and possesses but little appearance of fissile texture.
In the most Easterly tract of this formation, the denuda— C, 5. tions inland often appear in small rounded eminences, with smooth surfaces dipping gradually on one side, and presenting a vertical face on the other. With a little attention these are distinctly seen to be stratified in the manner represented (Pl. XVI. Fig. 4.), which is intended for a section of the Pillar rock near Plas-Newydd, and an eminence immediately on its N.E., when viewed from the East. This character prevails on each side of a line drawn from Llandonna to Newborough : but on the coast, from Beaumaris to Cadnant, the dip is Southeasterly.
Where a separation into laminae does not exist, the scales of mica or chlorite still preserve a degree of parallelism in the crystalline varieties, which, combined with the curvature of a stratum, produces irregular but parallel lines upon its exposed surface, whose general bearing is still towards the N.E. and S.W.
From these circumstances we may perhaps conclude, that wherever a laminar tendency is found in this formation, it was originally parallel to the planes of stratification. And here there appears to exist a marked difference between this and the quartz rock, in which it should seem, that the laminar tendency has arisen from an arrangement of the particles posterior to the present contorted position of the strata.
In endeavouring to account for any appearance exhibited by these rocks, it is necessary to take into consideration the more Vol. I. Part II. 3 B