صور الصفحة
PDF

vailing - character is that of an hornblende rock, composed of large crystalline plates interlacing in various directions, and cemented together by a little felspar and carbonate of lime (689.). The felspathic cement gradually increases (690.), till it forms a greenish compact basis, through which the crystals of hornblende are dispersed (691.). Other varieties present a more perfect intermixture of these two ingredients. (692–694), with the addition of small shining plates, apparently diallage (695.). White felspar and dark grey hornblende form also a finely granular compound, which resembles some varieties of the trap included in the dykes (696, 697). Distinct, green crystals of hornblende are embedded in a basis of crystallized white felspar, and it is worthy of remark, that some of these crystals have been broken, and the fragments lie in different directions, surrounded by the felspar, the edges of their corresponding extremities tallying with each other (699.). The greenstone to the East of Llanbabo is not well exhibited. Its characters are precisely the same as the former (712, 713.). Having proceeded thus far with the description of the granitic districts, before any attempt is made to establish the probable history of the rocks which they include, it may be here remarked, that the phenomena which accompany them are so very similar to those presented by the trap dykes, that we can have little hesitation in ascribing their origin to the action of the same cause. This circumstance is premised, that the object may at once be seen for which any particular appearance, tending to establish the theory of their common origin, is recorded. From what has been stated, it will readily be conjectured, that the theory alluded to is that which ascribes the formation of these rocks to the influence of volcanic action, and it must be

perfectly unnecessary to recapitulate the arguments which have Vol. I. Part II. 3 I

been urged by others in its support, and drawn from appearances similar to those described under the details of the several dykes enumerated in the preceding part of this paper. They are such as will suggest themselves to every one, and some speak so strongly in its favour, that it seems scarcely possible for the most sceptical on this head not to allow the force of their evidence. - In addition to those arguments which may be deduced from such phenomena, it may be stated, that the number of these dykes must be very considerable; for many of those enumerated have become exposed, by mere accident, in the different quarries opened for the purpose of repairing the roads, and it may reasonably be expected that there are very many others concealed beneath the cultivated surface, as well as several which have escaped observation. In no one instance does it appear, that they are in any way associated with a superincumbent mass of the same nature, and indeed the great variety of mineral character which they assume is alone a strong argument against supposing them ever to have formed members of a common body. None of the veins and fissures which contain them appear to terminate downwards, whilst on the contrary it should seem, that, in some instances, their termination upwards has been clearly ascertained. In others, there is every probability, that a considerable part of their course lies beneath the rock with which they are in contact at the surface. There is one argument brought against the igneous theory which may be supposed to derive weight from the investigation of Anglesea. This is, that the trap, if projected in an ignited state, would have produced results of a more -uniform character, whereas in many cases it should even seem that it has produced no alteration whatever upon the surrounding rock. Now, one decided example of alteration should speak more plainly towards establishing the nature of these dykes than any negative argument which might be drawn from those cases in which no such alteration is found to take place; for we know it has been determined by experiment that certain rocks, when fused, will afterwards return to their former state, if placed under those very circumstances which most probably must have existed at the time of their fusion. This fact may be illustrated by referring to the phenomena which accompany the dyke at Plas-Newydd. The alteration which there takes place in the surrounding rock, although of the most decided nature, is by no means uniform, even in the same stratum. On one side, we have a mass of soft clay shale assuming a hard jaspideous character, whilst, on the opposite side, this alteration is partial, and the rest puts on a crystallime structure; and intermixed with this we also find some portions in an earthy state. That the whole is not crystallized may readily be accounted for, by supposing a superabundance of calcareous and argillaceous particles, above the requisite proportion necessary for the formation of the crystals; still, however, it shews us that in the very spot where the change is the most marked, it is yet possible for some portion to remain unaltered. If the granite of Anglesea be justly ascribed to the same class of rocks as those which compose the trap dykes, it should seem equally certain, that some portions of it must either have resulted from the fusion of the surrounding strata, or else have been considerably modified by an intermixture with them, and consequently that it is more recent than any with which it is associated. At the South-western termination of the Northern granitic district, there is a patch of old red sandstone. Although the 0.4. whole of this appears to have been considerably changed from

its original character, and to have assumed a more compact and crystalline structure, yet at the furthest point West from the granite, it is evidently composed of sandstone mixed with coarse breccia containing pebbles of quartz and slate (784.). The strata run directly towards the granite, and several opportunities occur of examining the alterations which they sustain. Upon approaching the granite, the crystalline character increases, the materials become more firmly cemented, and pass into each other (785–788.), till at length, without any abrupt transition (789.), the strata merge into a crystalline rock (790–796.), in which the modular concretions of quartz have scarcely lost the aspect of pebbles (790.). Felspar, frequently of a talcose or steatitic aspect (792), forms the principal ingredient of the resulting granite (798—801.), which contains large, distinct concretions of quartz and mica. Through it there are also dispersed irregular masses of impure adularia, which cleave with great facility (797.). At the spot where the sandstone has first assumed the decided character of a granite, there occur a few specks of galena (796). A repetition of similar appearances may be traced along the boundary between the old red sandstone and South granitic district. Towards its Northern termination, immediately to the West of Llanerchymedd, the granite is found in the bottom of a valley which passes between two portions of the sandstone. At the last spot where its effects are distinctly marked, there is a quarry in which the rock may truly be said to constitute the intermediate passage between the two. The remains of a coarse sandstone are evident in some parts of the quarry, and the passage (777—780.) to a perfectly crystalline mass (781.) distinctly visible. The whole shatters into small fragments, and a considerable portion is converted to yellow ochre, which also coats over the natural cleavages in the more solid portions. A vein of crystalline quartz, one inch and an half thick, traverses the decomposing, porous portion of the rock (783.), and with it are intermixed irregular stripes of chlorite, which penetrate the quartz, and are so disposed, as to form rudely parallel lines inclined to the sides of the vein at an angle of 45° (782.). Where the chlorite is not present, the quartz still preserves a tendency to cleave in this direction; a circumstance which bears a striking resemblance to a fissile texture, oblique to the disposition of a bed. About one mile to the South of this spot, there is one of the localities already pointed out, for the fossils in the old red sandstone. The quarry in which they occur is on the confines of the granite, and the neighbouring mass of rock, which projects a few yards to the East, is in fact completely crystalline. Some parts of the quarry also approach the same structure, and a gradual obliteration of the fossils is the consequence. The impressions are coated with oxide of iron (369.), and as the matrix loses its original character, their position becomes marked only by an irregular cavity, retaining a partial impression of some portion of the cast (370.), till at length the spot where they formerly existed is simply traced by a shapeless ferruginous patch (37.1.). Having found the impressions of anomiae in the midst of the altered shale at Plas-Newydd, and even where it had assumed a crystalline structure, it did not seem improbable, that some traces of these shells might also be met with in the neighbouring granite, derived from the altered sandstone. But the search proved fruitless, and indeed we can scarcely expect that any such can exist. The less compact state of the sandstone, and the character of the resulting rock, so much more uniformly crystalline than that of the altered shale, would render it less likely that any appearance of this description could be preserved. It may seem singular that I should have searched, in the granite for a fossil, as a circumstance likely to increase the

« السابقةمتابعة »