Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/560

436 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 23, the syenite, in its main mass, nowhere comes into contact with the green slates and porphyries, but is everywhere surrounded by the upper beds of the Skiddaw Slates, which are often more or less indurated and penetrated by numerous small veins of quartz near the line of junction. It is well exhibited in a quarry about a quarter of a mile to the S.E. of Threlkeld Station, where it is so regularly jointed as to assume the aspect of a bedded rock, the joint-surfaces dipping E.N.E. at 50°. In appearance it is of a light greyish colour, some specimens being almost white. It consists of a felspathic base, enclosing numerous large crystals of a greenish-white felspar, with many small specks of hornblende and little masses of transparent crystalline quartz, together with occasional crystals of garnet. The rock has also imbedded in it a great abundance of angular fragments, some of which consist of trap, whilst others are laminated and almost gneissic, and appear to be derived from the Skiddaw Slates. On the opposite or western side of the Vale of St. John the syenite forms the whole of Low Rigg, and can be traced southwards as far as the chapel of St. John, at which point it is overlain by a green felspathic trap, which forms the base of the green slate series. No alteration, however, is observable in the trap near the line of junction. It has been largely worked near Hollin Root, where it is very regularly jointed and possesses the same mineral characters as near Threlkeld Station, except that in parts the base becomes red, and its aspect thus becomes more decidedly syenitic.

About a mile and a half to the east of the Vale of St. John, close to where Mosedale Beck crosses the road between Matterdale and Threlkeld, there occurs an intrusive mass of felstone, which is undoubtedly an extension of the syenite seen near Threlkeld Station. It is a greyish-white, granular, felspathic rock, in parts very regularly jointed, often containing cubes of iron pyrites, and rarely exhibiting small masses of quartz. As the quartz, however, is very sparingly developed, and the hornblende has entirely disappeared, it can no longer be called a syenite, but must be looked upon simply as an occasionally quartziferous felstone. On the north and north-west the upper shaly beds of the Skiddaw Slates come into contact with this intrusive boss, and are somewhat decolorized, and even slightly brecciated near the line of junction. To the south the felstone is overlain by the trap which forms the base of the green slate series, but without the production of any perceptible alteration.

In connexion with the syenite of the Vale of St. John, I must allude to a remarkable felstone dyke, which is apparently derived from the syenite. The dyke in question was first noticed by Professor Sedgwick (see 'Letters'), and occurs about a quarter of a mile above Armboth House, in the course of a stream which flows from the west into Thirlmere Lake. It cuts across a series of bedded traps, which form the lower part of the green slates, and appears to strike N.W. and S.E., being seen again close to the Ordnance Cairn, nearly half a mile to the S.E. of its exposure in Armboth Beck. In mineral characters it is very peculiar, consisting of a base of reddish-brown felspar containing numerous large oblong