436 PEOCEEDI]!fGS OF THE GEOLOGICAL SOCIETY. [JunC 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 indu- 

 rated 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 Yale 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 junc- 

 tion. It has been largely worked near HoUin Eoot, 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 un- 

 doubtedly an extension of the syenite seen near Threlkeld Station. 

 It is a greyish- white, granular, felspathic rock, in parts very regu- 

 larly 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 Yale 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 Pro- 

 fessor 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 



