Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/836

 predominating. These beds form the whole of Brackenriggs Fell as far as Shoulthwaite Gill. They vary much in character, being sometimes compact and fine-grained, at other times highly porphyritic. In places the bedding is well shown, the dip being S.S.E. at 25° to 30°. At Shoulthwaite a synclinal fold occurs ; and in proceeding up Shoulthwaite Gill we have a repetition of these bedded traps, with a few ashy and brecciated beds, the sheets of trap dipping northwards at angles of from 20° to 30°. This northerly inclination is particularly well exhibited in the flanks of the Benn and Low Bank, on the eastern side of the valley, and of Castlerigg Fell and Bleabery Fell, on the western side.

IV. Lower portion of the Green-slate Series in the Vale of St. John.

The north and south ridge which separates the valleys of Saddle Beck and the vale of St. John, exhibits a very clear section of the lower portion of the Green-slate series (fig. 3). The extreme northern end of this ridge, known as Low Rigg, is entirely composed of the intrusive syenite which I have described as occupying the mouth of the vale of St. John (Quart. Journ. Geol. Soc. vol. xxv. p. 435). Southwards the syenite is succeeded at the Chapel of St. John by a compact, fine-grained, dark-green, felspathic trap, which forms the northern end of Naddle Fell, and is well seen above the farm of Rake How. This trap is the lowest member of the Green-slate series ; and it forms a tabular mass of considerable thickness, which dips S.S.E. at about 30°. It is not, however, succeeded by ashes and breccias, as in Borrowdale and at Keswick ; but it is overlain by a great series of felspathic traps and greenstones, which form the whole of Naddle Fell and High Rigg, on the western side of the Vale of St. John. Occasionally there occur thin bands of ash, as at Sosgill and Low Bridge End ; and at this latter place the ash is highly amygdaloidal. This succession of bedded traps is exceedingly well displayed along the flanks of Naddle Fell, forming a series of oblique terraces, which dip S.S.E. at about 30° ; and a similar sequence of beds, though not in so marked a form, appears to obtain in Wanthwaite Crag, on the eastern side of the valley. The traps of Naddle Fell vary a good deal in lithological characters, being usually more or less porphyritic and containing a considerable quantity of hornblende. At the southern end of Naddle Fell there is a highly porphyritic trap, which closely resembles some of the beds seen near Aira Force ; and this is overlain in Warren Crag by a great mass of greenstone, which seems to be a continuation of the celebrated Castle Crag, on the eastern side of the valley. If this is the case, however, it is shifted to the north by a fault. The whole series of traps is succeeded close to Smeathwaite Bridge by bedded felspathic ashes, which dip S.S.E. at 45°. Continuing the section from Smeathwaite Bridge down the western side of Thirlmere, we cross again a similar series of traps and greenstones to those which occur in the Vale of St. John, the repetition being doubtless due to the occurrence of a synclinal, though it is extremely difficult to make out any dips.