THE SHAP GRANITE AND ASSOCIATED ROCKS. 289 



quartz— ^.^. the dyke between Crookdale and Borrowdale [1103] ; 

 while others, again, belong to intermediate types — e. (j. the dyke or 

 series of dykes on Potter JFell [792]. In this latter rock, at a 

 distance of six miles from Shap Fell, are found long flakes of dark 

 mica with the blade-like habit of that noticed in some parts of the 

 granite margin. 



None of the various intrusions we have alluded to can be traced 

 as continuous with the granite at the present surface. If we are 

 right in regarding them as apophyses, they are in connexion, not 

 with the visible granite mass, but with a deep-seated extension 

 of it. 



The small dykes found in close proximity to the margin of the 

 granite outcrop, such as the two seen not far w^est of Wasdale Head 

 Farm, are ordinary quartz-porphyries with no special peculiarities, 

 except that they sometimes contain little crystals of brown sphene 

 [757], an nncommon mineral in such rocks, though it might almost 

 be expected in any offshoot of the Shap Eell granite. The same 

 constituent occurs sparingly in the brown-coloured rock with por- 

 phyritic quartz and felspars which is prominent in the Blea Beck 

 section, apparently forming an irregular sill at the summit of the 

 Lower Limestone [882]. 



There is no doubt that many dykes which we have failed to 

 observe exist to the north of Shap Fell. Such dykes would be 

 less noticeable in the Yolcanic Series than in the Silurians to the 

 south, and the quarter-sheet of the Geological Survey map for the 

 northern part of this district is not yet published *. Sedgwick 

 found a quartz-porphyry not very different from that of Blea Beck in 

 Wet Sleddale [803]. This may be a dyke or sill belonging to the 

 inner group, connected at or near the surface with the Shap Fell 

 granite. A large djke belonging to the outer group of apophyses 

 cuts the Skiddaw Slates at Goodcroft Farm, near Eossgill. It is 

 about four miles north of the granite outcrop, towards which it 

 bears directly. It is twenty to forty yards wide, and encloses 

 entangled masses of indurated slate. The rock is a normal quartz- 

 porphyry [1164]. 



When these apophyses are considered in conjunction with the 

 patches of darker rock caught up in the mass of the granite, they 

 appear to throw light upon the origin of several of the dykes which 

 penetrate the Lower-Palseozoic rocks of the district, and which are 

 thickly clustered in some areas, whilst they are much rarer in others. 

 They abound within a radius of fifteen miles of the Shap granite 

 (see Map, fig. 4, p. 290), whilst others are found in great numbers 

 around the other granite areas. These latter are usually felsitic, 

 whilst those more immediately in the neighbourhood of the Shap 

 granite are both felsites and mica-traps, and, so far as we are aware, 

 the latter are chiefly confined to the east end of the Lake District, 



* [Since this paper was read we have, by kind permission of the Director- 

 General, compared our map with the six-inch MS. map in the Survey Office, 

 and inserted several additional dykes in the country north ot the granite. — 

 March 11th, 1891.] 



