at an angle of 45°, and exhibit a fault by which they are upheaved towards the schist. Similar appearances to these may be seen in different quarries between this spot and Trefdraeth. At Bodorgan, an isolated patch of schist rises through the confused and dismembered grit. Between the grit and schist there is a loose breccia chiefly composed of angular fragments of the latter (392.), which may be accounted for in the same manner as the breccia interposed between the clay slate and mica slate on the N. W. side of the Llydiart mountain. On the W. of the Pentraeth river, near Red-wharf bay, the limestone dips 45° to the W. of N. In Pentraeth their inclination reaches as high as 80°. At Caint they are confused, broken, and sometimes contorted without fracture, Pl. XVII. Fig. 2. But on proceeding to the West of these several places, we find the strata nearly horizontal. They present several low cliffs, which are not so abrupt as those on the Western boundary of the series. These facts seem to indicate, that the grit and limestone terminate abruptly against the schist to the East, with the intervention of a few hundred yards of disruptured and broken strata. East of Llandonna, the limestone presents an abrupt cliff M. 3. to the sea, the strata are nearly horizontal, their edges reposing on an inclined plane, the summit of which is chlorite schist; but at a lower elevation we find the shattery schist before-mentioned, so that the limestone overlaps this in the manner represented Pl. XX. Sect. P. Large fragments of the limestone strata are scattered over the steep sides of the chlorite ridge between this spot and the sands of Red-wharf bay. This district extends to the East as far as Priestholme island. Near the point to the North of Penmon, some coarse grit sets on. The dip is towards the E. of N. at no great angle or inclination. M. 2. M. 2. The appearance presented by Great-Ormes-Head Pl. XVII. Fig. 3. on the opposite coast of Carnarvonshire is such as might lead us to expect a continuation of the same strata at that place. I did not visit the spot, but it is evident from the opposite coast, that a considerable indentation Northwards takes place towards the Eastern extremity of the Head. This must expose each stratum at some point further to the North than its Southern boundary, where (owing to the former dip towards the E. of N.) it will be seen at a less degree of elevation, which would give rise to the deceptive appearance of bason shaped strata exhibited towards the East of the figure. In the tract lying to the S. W. of Bangor, the strata belong to the coal-measures. A little to the South of the spot where they first appear, there is a large limestone quarry, which lies beneath some beds of grit and shale, and is possibly a portion of the series belonging to the mountain-lime. The strata here also dip towards the E. of S., and are in contact on the East with greywacké, and the older rocks dipping also, when laminated, in nearly the same direction. The line of junction is obscured by a cultivated valley. They are bounded by mica and chlorite slate on the West. The fossils found in this formation are anomiæ, madrepores, trilobites, and others identical with those from the mountainlime of England (405–421.). Magnesian Limestone. {Nos. 422 to 474.} To the South of Plas-Newydd park, there commences a series. of limestone and sandstone strata, which overlie the coal-measures, Map. This place lies to the East of Priestholme island, but is without the limits of the and appear to belong to a separate formation. They are better exposed, and may be examined with greater convenience on the opposite coast. The lowest portion consists of rolled fragments of limestone, cemented together by argillaceous and calcareous matter (422.). To this succeed beds of limestone, grit, and sandstone, variegated with deep yellow and brick red colours. Their order from the bottom is, 1. Yellowish brown sandstone (424.) 2. Compact and crystalline grey limestone, with specks 3. Bluish shale, (thin bed) 4. Compact flinty dark grey sandstone, nearly a pure 5. Fine red, striped sandstone, (442.), containing frag- The two last beds contain variable portions of lime. 6. Thick bed of compact red limestone (427, 428.), which has been quarried to a considerable extent. Upon this are imposed other strata of a similar nature to those described, whose order of superposition it is not so easy to ascertain. They are all more or less characterised by containing beds of bitter spar (437-440.). The fossils are generally in an imperfect and shattered state, intermixed with pebbles (434, 435.). The more perfect madrepores are frequently traced in deep red upon a light ground (432-434.). These fossils appear to have belonged to the mountain lime, and may be considered as embedded fragments in the present formation. Although I found no good section, by which any positive information might be, obtained of the nature of collocation between this series and the last, still it seems probable, that they lie unconformably to each other. Immediately South of PlasCoch, the black limestone and shale of the coal-measures dip towards the E. of S., and a few yards to the East of this spot, the red beds of grit are found dipping in a contrary direction. On the opposite coast, to the East, of Plas-Newydd, the lowest strata of the red grit and limestone dip gently to the E. of S., apparently conforming to those of the coal-measures; but suddenly their dip is considerably increased, as if they were reposing upon the brow of a steep hill. As the red beds appear to be entirely wanting over the marshy tract in which the coal of Anglesea is situate, it is not unlikely that in the present place they overlie a considerable body of that formation. New Red Sandstone. {Nos. 475 to 482.} Over the strata of the last series, there occurs a rude mass of argillaceous and sandy materials (475.) intermixed with large fragments derived from the older rocks. The basis is occasionally consolidated into thin laminæ, giving rise to a slight appearance of stratification. The whole is of a deep red colour. It commences a little South of Moel-y-don ferry, on the Eastern side of the Menai, and extends as far as Carnarvon, but in Anglesea it forms only a small hummock on the North of Tan-y-voel ferry. Both this and the preceding formation terminate to the E. and W. in the same abrupt manner as the coal-measures. Many of the fragments dispersed through it, are of a large size, and generally consist of quartzose materials. Some are of grit in which the fragments run together and pass to a homo geneous quartz rock (476, 477.), others approach chert (478.) or hornstone (479–481.), and nearly all are tinged red. A fault in the coal-pit near Llanfihangel East, contains fragments of quartz intermixed with red sand (482.), and may probably have arisen from a portion of this formation having filled up a fissure. Trap Dykes. Although these form but inferior members among the unstratified rocks, still it seems advisable to commence this part of the account with their description; since the facts which they present tend materially to confirm the conclusions drawn from phenomena observed in more extensive districts, apparently of similar origin. As their number is very considerable, and as a detailed description of each would only occasion repetition, a selection has been made of those which are accompanied by appearances of the greatest interest. Where their course is sufficiently exposed, it is represented on the Map in the usual way, by a particular colour placed between parallel lines. But there are many slight indications of trap, where it is either impossible to trace the course of the dyke, from the concealed nature of the ground, or it only presents an isolated mass rising through the schist. In these cases, the locality is marked by placing a (7) as near the spot as possible. References of this description will seldom guide a second person to the dyke, but may serve a different purpose, and shew the number and relative situation of the places where trap was actually met with. Mr. Underwood has submitted specimens of several of these dykes to the examination of Professor Cordier, whose method of analysing the basalts, and accurate knowledge of their mine |