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tion is in their smoother surface, and more angular fracture: but this will frequently escape observation where a moment's inattention may carry us across one at a single step. It will be seen in Pl. XVIII. Fig. 4. at a and b, that there is a deceptive appearance as though these dykes terminated abruptly downwards; but in these cases the course may be considered as tending upwards, obliquely to the plane of the paper, when placed vertically, and coming from some point behind it. The deception arises from the face of the cliff intersecting the inclined side which bounds the furthest extent of the dyke to the East, a fact which I verified in one instance, by removing the schist from below. There is a marked distinction in this apparent mode of termination, and that which is seen in Pl. XVIII. Fig. 2. at a. In the latter case, the fissure containing the basalt, gradually becomes thinner towards the end, in the former, the entire width is preserved. The specimens examined by Professor Cordier, from this neighbourhood, were “dolerite. The pyroxene very evident, with fer titané.” “A basaltic lava, but more felspathic than the others. The felspar has the filamentous character of volcanic products, resulting from the crystals being flattened. To see this, two sides of the specimen must be placed at right angles to each other.” The appearance of the flattened crystals is common to several of the very compact dykes, and may be seen in some parts of the one near Cadnant, towards its Western termination. In the small dyke Pl. XVIII. Fig. 2. a, these crystals are few, and extremely minute (642.), the basalt being more remarkably fine grained and tough, than any other which I met with in Anglesea. An evident intermixture often takes place, between the trap and the surrounding schist, along the line of junction, which
sometimes resembles the gradual blending of two different colours in a mass of striped jasper (642.). Small portions of schist are embedded, near the sides of the dyke, which intermix with the trap, and modify its appearance and composition (641, 643.). The schist, in contact, has frequently a blistered aspect, with irregular cavities and flaws (644, 645.). Dykes immediately to the North of the Menai bridge. “Dolerite with superbe pyroxene.” Prof. Cordier. “Felspar and pyroxene with crystals of pyrites (665.). The circumstance of having crystals of pyrites, though rare in streams of basalt, is easily accounted for in a dyke. The extended surface presented to the air by the stream, would enable the sulphur to evaporate, but in the dyke it is condensed. Perhaps also the dyke never came to day.” Prof. Cordier. The presence of pyrites, frequently in the form of distinct crystals, is common to most of the dykes in Anglesea. Dykes to the South of the Menai bridge. “Basalt very rich in felspar.” Prof. Cordier. “Basalt poor in pyroxene.” Prof. Cordier. On the South-western coast near Aberfraw. “Plus travaillée than the other dykes—blistered.” Prof. Cordier. The passage to the earthy traps is perfectly insensible, and portions of the most genuine basaltic dykes are frequently of this nature. At Llangwyfan—“ Wacke endurcie. It is full of green earth, and ought to become cellular in an acid.” Prof. Cordier. Most, if not all, of the varieties of trap included in the dykes of Anglesea are occasionally amygdaloidal and porphuritic. Some contain nodules of crystallized carbonate of lime, which do not always exhibit the usual appearance of a rhomboidal cleavage common to the whole nodule, but possess an
uneven fracture, although the specimen is perfectly pellucid, approaching the character of saccharine marble (658, 659.). Embedded crystals of felspar are more common in the compact and earthy traps (661.), than in the crystalline (660.). The compact portions, of several dykes, assume a confused appearance of crystallization, and break into small fragments, a few inches in diameter, bounded by perfectly smooth surfaces. Several of these form accurate rhomboids (676.), others exhibit this figure modified by a diagonal cleavage (675, 677.); but it generally happens that their figure is less regular, and that no two faces are parallel to each other (678, 679). The effects of decomposition frequently extend to a considerable depth in the dyke, and we find each of these fragments, partially decomposed, presenting a portion of unaltered trap in the interior (680).
A rock formed of quartz felspar and mica, is found in each of the tracts laid down as including the granite; but the mineral character of the whole district is far from uniform.
In the Southern portion, about the neighbourhood of Gwalchmai and thence towards Llanerchymedd, the surface is broken by small detached rugged eminences, rising through a marshy
ground, which is bounded East and West by an abrupt
termination of the stratified rocks. The external character of all these protruding masses is so very similar, that it is impossible to calculate beforehand on what may be the real composition of any one in particular. On examination they are found to vary extremely; one may be a true granite, the next a pure quartz, the third a greenstone, &c. A better motion of this variety of composition may be obtained by referring to some of the specimens which were procured in the neighbourhood of Gwindu, within four miles North and South of the Inn. Among the granitic rocks the quartz is generally, white; the felspar is either white (724, 725.), brownish yellow (726, 727.), or flesh-red (728–737.); the mica silvery white (725.), black, or green (730–732). In the latter case it becomes associated with chlorite, which in many places entirely supersedes it (739, 743.), tinging both the quartz and felspar of a greenish hue (740). The chlorite also mixes with hornblende (741–744), and these two substances frequently predominate so much as nearly to obliterate the quartz and felspar (753, 754). Sometimes the felspar, of a flesh red colour, forms the basis of the rock, and the other ingredients are sparingly dispersed through it (728, 729.), (745, 746.). In other places, chlorite and mica supersede the rest (752.), and we then find only patches and veins of felspar and quartz, completely enveloped in the more trap-like rock (750, 751.). A beautiful variety is composed of dark green hornblende crystallized in large plates, and intermixed with irregular patches of white felspar (755.), which however frequently assumes a greenish tinge (756.). At the same spot there are patches of crystallized carbonate of lime penetrated by yellowish green spiculae of epidote (757.), a substance pretty generally diffused through the surrounding rock, either in veins (758.), or interlaminated with the hornblende (759, 760.). It occurs also in compact masses, intermixed with quartz (761). Patches of genuine basalt are scattered throughout the district, completely enveloped by the granite, and possessing the same character as the trap found in the dykes of various other parts of the Island (762, 763.). All these varieties are highly crystalline; but with them we find rocks of another description, whose composition is more
nearly homogeneous. They possess a flinty aspect approaching to hornstone, and are of various shades of white (766, 767), grey (768.), or green (769.). Here and there a crystalline structure is exhibited, or a few crystalline specks lie dispersed through the compact base. This variety in the mineral composition is chiefly confined to those parts of the district which present a broken rugged outline. In the elevated ridge which stretches from Gwalchmai to Lanfaelog, the character is more uniformly granitic and the surface of the ground unbroken. The quartz and red felspar have not the distinctly granular appearance which they generally assume in substances of this nature, but are intermixed with a more pasty aspect than usual (734.), and the lustre frequently deadened by a superabundance of the oxide of iron (737.). The Northern district occupied by the granite is not so variable in its character; the usual appearance being that of an irregular and large grained intermixture of quartz, white felspar and silvery mica. A greasy lustre is frequently given to portions of this granite, which apparently arises from its being contaminated by a considerable quantity of talc (790–801.). By referring to the Map it will be seen that there are two districts which consist entirely of greenstone. The general character of the rocks which compose them, is so nearly allied to some parts of the granitic district to the South of Gwindu, and their relation to the surrounding strata so very similar, that little doubt can exist of their belonging to the same formation. The district to the North of Llanerchymedd is marked by rugged, and rudely shaped masses, projecting through the surface. These extend from a spot about one mile to the North of the town, on the West of the road to Amlwch, towards the North of East, and pass a little to the North of Llandyfrydog. A pre