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number of arguments in favour of its igneous origin. But what has been stated may serve as a caution against forming any hasty conclusion to the contrary, should such a discovery be ever made. The altered appearance of the old red sandstone, which lies to the West of the Southern granitic district, was remarked in the description of that rock. The facts which have been just stated seem to point out a cause adequate to the explanation of this circumstance, and there are besides some other particulars connected with its history, which tend materially to
confirm this supposition. From the lake at Llanfaelog, to
Llanfihangel, the surface is swampy and uncultivated, through which many masses of bare rock project. Several of these present an aspect so highly crystalline, that at first sight a question might easily arise, whether we were not still in the midst of the granite, until a second blow from the hammer clears up the doubt, by exposing a mass of hardened sandstone. In short, the state of this sandstone appears to be only a degree removed from the more crystalline structure of the granitic district which lies to the North of Gwindu. Near Llanfihangel church, on the South, and in the midst of an assemblage of rocks distinctly composed of the brecciated materials, we find a mass of trap (804, 805.). The felspar is sufficiently distinct, and forms the chief ingredient in the basis of the rock, through which a few embedded crystals of the same mineral are scattered, giving it a slightly porphyritic character. The whole assumes a greenish tinge, but the colouring substance does not appear to be of a very crystalline nature, and is probably chlorite. This intermixes with a confused aggregation of hornblende and diallage (807, 808.), passing by insensible shades to the breccia which surrounds it. In the very midst of the more crystalline portion we find small patches possessing a trace of the original character not quite obliterated (809.).
The rock often passes also to a light green felspathic mass, spotted or mottled with dark green. Several such appearances occur in the form of smooth nodules, already alluded to as embedded pebbles, but I strongly suspect that they must be of a concretionary nature, similar to those in the steatitic rock near Bangor. The whole of the exposed rock to the S.E. of Llanfihangel church, has more nearly the external character of a mass of trap than of any other substance. It possesses no traces of stratification, but is rent by fissures which divide it into prismatic and rhomboidal blocks. One of these is so singular in its appearance, that I have given a sketch of it, Pl. XVIII. Fig. 5. It resembles a basaltic column lying upon its side, and is composed of felspathic and chloritic matter, mottled and blended together (810, 81.1.). On the N.W. of the lake near Llanfaelog, there are several instances of a similar passage of the breccia to a trap rock (812.). This apparent conversion of the schistose breccia, belonging to the old red sandstone formation, to a trap rock, seems more distinctly to connect the greenstone with the granite, and to point out a common origin for the two, which also receives additional confirmation from the examination of the tracts occupied by the former rock. The patch of greenstone to the North of Llanerchymedd is surrounded by greywacké, the basis of which is a glossy black clay-slate. In the immediate vicinity of the greenstone, this greywacké is curiously affected; the embedded fragments of schist assume a yellow decomposing tinge, whilst the quartz becomes more crystalline (701–705.). The next step presents a rock of decomposing aspect, through which are scattered traces of crystalline structure, resulting from an imperfectly formed hornblende, mixed with felspar (706– 710). The latter is distinctly marked, but the crystals of the former bear a strong resemblance to fragments of slate. They are frequently broken transversely, a circumstance which it has been stated also occurs in the genuine crystals of the same substance in the neighbouring hornblende rock.
It does not appear very evident why hornblende should here result from the fusion of schist, and that pyroxene should be a constituent of the dykes which are presumed to be of similar origin. There is, however, one point of difference between them. In the dykes, the fused matter appears to have been injected into a fissure of the superincumbent rock, but in the present instance the alteration has taken place without any progressive motion. There are other rocks, in this part of Anglesea, of which hornblende is an ingredient, where the transition from the schist to the trap is not marked by a distinct line, and where a similar explanation might be given of their origin.
Near the summit of Llaneilian mountain, towards the South, we find masses of this rock, protruding through the greywacké, in which the hornblende is sometimes well crystallized (715.), and at others scarcely to be detected (716).
At the bottom of the cliff, to the N.E. of the highest point of this mountain, a similar rock is found, but the hornblende is not so distinct as in the former case (717.). Upon ascending the cliff the appearance of a dyke is gradually lost, and it scarcely exhibits a structure sufficiently crystalline to separate it from the schist (719—722). Through this dyke there run several veins of quartz, which also abound in the surrounding rock, a fact which I do not recollect witnessing in any other dyke in Anglesea. Irregular strings of reddish compact felspar, of a cotemporaneous character, are also found in it (718.). The schist in contact is a fine grained clay-slate (723.), and in the dyke there occur several strings, or thin laminae, of clay-slate of the same hature. -
Patches of glossy crystalline clay-slate are also found among the hornblende rocks to the North of Llanerchymedd. On the South side of the road from Llanerchymedd to Llechynfarwy, before quitting the former place, there is a quarry which partly consists of clay-slate, and partly of hornblende rock and greenstone, similar to that on the North of the town. Considering the extensive influence which must have been exerted to form the granitic districts, we might also expect to find the rocks in their vicinity modified by its action. Where the South granitic district joins the older rocks to the East, it is not so easy to ascertain when an alteration has taken place ; since we are not always certain of the original character which these rocks themselves possessed. In some places, however, there appears to be little doubt of the fact. On the sea-shore, immediately South of Llanfaelog lake, the confusion and alteration impressed upon the schistose rocks are of a very marked description. They vary in composition and aspect at every step, and have scarcely a trace of fissile texture remaining. There are slight appearances of a crystalline rock, resembling some varieties in the granite round Gwindu (824.), but the general character is that of an homogeneous flinty mass of different shades of green (825, 826), grey (827), and brown (828, 829.). Since all these will fuse before the blowpipe, though with difficulty, they seem to approach the character of a hornstone. One of the specimens presents a singular fact, and as the experiment was several times repeated, there can remain no doubt of its accuracy (827.). It forms a dirty white mass between compact and finely granular, and seems to consist principally of quartz, but contains also a little lime disseminated through it. A few faint streaks of green matter, resembling chlorite, are intermixed with the substance of the stone. When the specimen is exposed to a red heat, the Vol. I. Part II. 3 K
green veins immediately turn jet black, assume a laminated texture, and strongly resemble pyroxene. It then fuses, with some difficulty, to a black glass. There are some portions of a more compound nature (830.), intermixed with the homogeneous rock, which appear to be composed of small fragments of quartz firmly embedded in a paste of the same substance (831.), and others in which the embedded fragments are so loosely set, that they might be detached (832, 833.). With these are associated patches of blistered schist, gradually blending itself with the compact mass (834.). Proceeding Eastward along the shore, we find traces of a laminated texture making its appearance. The whole rock still forms a flinty mass, but the smooth surfaces exhibit parallel contorted lines of obscure yellow upon a green ground (835–837.). This character prevails until we arrive at Llangwyfan, after which the rocks become more regularly schistose. There are numerous trap dykes scattered throughout thes whole of this district (813–823.). These vary considerably in character; some form a perfect basalt highly charged with crystallized carbonate of lime, tinged green (813), but the generality, are of a more earthy mature, and vary through different shades of dark grey and green. In texture and composition, they often resemble clay-slate so closely, that a detached specimen might easily be mistaken for this rock (820–822.). They are generally porphyritic, containing embedded crystals of felspar, and alter their character completely, and suddenly, through different parts of their course. Many of them are seen to ter— minate in both directions, and some form mere bumps rising through the hardened schist, and are themselves again intersected by smaller dykes of a different character. There are several other appearances of a similar description impressed upon the schist near the granite, both in the neigh