elevation than the surrounding country, and in the first-mentioned situation forms the highest point of all Anglesea.

On the summit of Holyhead mountain, and on the highest point near Rhoscolyn, the term given to this rock is strictly applicable, it being composed of little else than highly crystalline and distinctly granular quartz (1.) firmly cemented. There occur in it a very few minute white specks of earthy felspar. It is much intersected by cotemporaneous veins, and occasionally tinged red (3.). In other places it intermixes with a little mica (4. 5.). The quartz is often finely granular (6.), and associated with larger and distinct crystals of silvery white mica (7.8.), and apparently also with a little chlorite, the specimens assuming a greenish tinge. Such specimens strongly resemble greywacké, but their crystalline nature is still very distinct.

With regard to the structure of this rock, nothing can be more deceptive than the appearance which it assumes in places Q.1. where no extensive section exists. On crossing Holyhead mountain, we seem to be walking over the edges of parallel strata, which dip at a very high angle towards the N. of W., the whole surface consisting of broken rugged lines, running from S. of W. to N. of E. Opposed to the small island called the South-stack, is a perpendicular cliff of two or three hundred feet in height, exhibiting the structure of the mountain in a perspicuous manner. Every trace of the former apparent disposition of the strata is lost, and the whole is seen to consist of broad strata, contorted in a most extraordinary manner, often vertical in position, then returning with a sudden curvature, and forming repeated arches. Strings of white quartz, which occur in and between them, partake equally of these contortions, and also of others more complicated and independent of the general position of the surrounding mass. The effect is rendered still more striking by each stratum assuming a peculiar tint; the colours varying through obscure shades of green, brown, and yellow.

They also vary in texture, which causes the more compact portions to project in relief, and these in weathering exhibit convolutions in which the softer strata do not partake. They are sometimes divided by fissures, generally placed nearly at right angles to the curves, producing the effect of an artificial stone arch. Plate XV. represents about fifty feet perpendicular height of this section. Wherever a convenient opportunity occurs of examining the cliffs along the remainder of this district, the same appearance is repeated.

Near Rhoscolyn the strata are very distinct, and among Q. 2. them is one of a brick red, contrasted with others of a deep yellow colour. The same deceptive appearance of stratification running from the S. of W. to the N. of E., is seen in this district as well as in the former.

The real structure of this rock, then, consists of a succession of contorted strata rudely conformable to each other. That these were originally deposited in their present position seems impossible, and the whole bears a striking resemblance to the flexures that might be formed in a pasty unconsolidated mass, by the application of a disturbing force.

The deceptive appearance resembling stratification, arises from the parallelism preserved between the scales of mica dispersed through the mass, which causes an imperfect kind of cleavage throughout the whole district. In some cases this may readily be exhibited, even in hand specimens (6.), where the mica has a brown ferruginous aspect, coating over the whole surface produced by cleavage, with a plate too thin to be detached. These laminæ occur about one eighth of an inch asunder, and on fracture exhibit an uneven undulating surface. A few other small scales of white mica are irregularly dispersed through the specimen. In some cases (4.) the laminar tendency is distinct, with the intervention of a very small portion of mica, Vol. I. Part II.

3 A

and on a fracture perpendicular to this direction, its existence is marked by faint lines. In other instances the cleavages, sufficiently apparent on the large scale, would not be noticed in a small fragment. Here (7.) the mica which gives rise to them is dispersed at intervals over their surface, and is scarcely 'to be seen upon a transverse fracture.

Other cleavages exist in the mountain, at much greater intervals than the former, which more nearly resemble natural fissures. These present smoother surfaces, and are probably the cause of large vertical fissures which occur towards the summit of the mountain, separating the rock into rude rhomboidal and cubical masses, where quartz is almost the sole ingredient, and both the fissile texture and divisions of the strata are scarcely to be detected, though, in convenient situations, each may occasionally be traced.

The exposed faces of these masses (2.) present an even polished surface, the effect of weathering; an action which apparently soon ceases, the fragments retaining their angles as sharp as when first fallen.

Plate XVI. Fig. 1. represents the positions of four cleavages obtained from a projecting mass of curved strata (6.).

A. The curved surface of the stratum.

Plane 3. is the cleavage which produces the apparent stratification of the whole mountain.

It should seem that these phenomena arise from some effort of crystallization subsequent to the original deposition of the

materials, and subsequent also to the present contorted position of the strata. These facts may be illustrated by the appearance presented upon the transverse fracture of a calcareous stalactite, where the original structure arising from successive depositions, is exhibited by concentric circles, whilst a rhomboidal fracture marks the effects of a posterior crystallization. The strict resemblance which some of the strata bear to those of sandstone, points out a mechanical deposition as the most likely mode of formation their present structure may suggest an idea that crystalline force assisted by moisture and pressure is an agent of sufficient power to have produced the similar, but still more perfect texture of the oldest stratified rocks, without the necessity of imagining any previous solution of the ingredients which compose them.

Chlorite Schist.

{Nos. 12 to 187.}

Under this denomination are included several varieties of schist, of which quartz and chlorite form the principal ingredients. Mica slate and clay slate also occur in the same formation, but each of these passes into chlorite schist by insensible gradations, and I could no where trace a boundary between them, marked either by a rapid change in the mineral character, or by some distinct geological feature. The varieties of clay slate included in this formation appear to consist of nearly the same ingredients as the chlorite schist, and to differ from them in nothing but want of crystalline structure. They are of various shades of green and red, and of a close texture.

The variety which immediately succeeds the quartz rock is crystalline chlorite schist. There are four sections on the coast

3 A2

C. 1. exhibiting their union. At the spot where this takes place between Holyhead and the mountain, there is a space of eight or ten feet in width, occupied by a rock intermediate in character between the two. At the spot on the beach where the change has become decisive, there occurs a breccia (9. 10.) of quartz rock, and angular fragments of talcose slate; from this a vein issues, which may be traced on the shore for a few feet as far as the cliff, up which it is seen to rise, and become forked near the top (Pl. XVI. Fig. 2.). The vein consists of finely granular materials (11.), with occasional patches of the breccia from which it proceeds. Possibly, the origin of this vein may be ascribed to a fissure in the chlorite schist having been filled from the subjacent rock, previous to its consolidation, by the force of the superincumbent pressure.

C. 1.

At Port Dafreth, I could not ascertain whether any sudden change takes place; the strata of the quartz rock gradually beQ. 2. come thinner, and appear to pass insensibly to chlorite schist. the West of Rhoscolyn their boundary is more marked, the quartz rock rising nearly vertically, and the chlorite schist resting unconformably against it. A small bay is there formed by the removal of the chlorite schist, so that its Eastern side is composed of quartz rock alone, where a broad stratum presents an undulating and nearly vertical cliff. At certain points of projection, a portion of this is removed, which exposes the stratum next below; a similar exposition of the next takes place, and so on. The nature of this may perhaps be better understood by referring to Pl. XVI. Fig. 3.

The last spot where this junction takes place lies to the South-east of Rhoscolyn, and West of Borth-Wen, where the confusion is greater than at either of the three other places. The quartzose strata intermix with the chloritic, and are seen on the beach like veins twisting among them. In some instances they even appear to occur in an inclined position above them.