part 2] THE SlIAP MINOR INTRUSIONS. 123 



anatase, and (in one section) allanite are also found as accessor}' 

 minerals. 



Examples of the earlier or orthoporphyritic dykes occur near 

 the granite and on the moors to the south as far as Borrowdale. 

 The rocks are pale grey in colour, highly porphyrinic, and the 

 ground-mass is microgranular. The abundance of phenocrysts 

 frequently gives to them an almost granitic appearance. Apart 

 from the large orthoclase-felspars, numerous smaller phenocrysts of 

 white or yellowish-green striated felspar, abundant quartz, and 

 biotite in varying amount, with an occasional plate of muscovite, 

 make up the bulk of the rock. In thin section, quartz, oligoclase, 

 and microperthite are the usual porphyritic minerals, the relative 

 proportions varying somewhat in different intrusions. The oligo- 

 clase may have fairly large extinction-angles, while microperthite 

 more irregular in outline, besides enclosing the former in parallel 

 growth, may either be in tergrown with quartz, or fringed by micro- 

 pegmatite. The large granitic orthoclases are also perthitic in 

 places, most often marginally but sometimes internally, especially 

 round the irregular patches of plagioclase and quartz. The perthite 

 is probably of the secondary type. 1 Apatite, zircon, and pyrrhotite 

 are fairly common accessories ; but sphene, on the whole, is rare. 

 Towards the edge of an intrusion the phenocrysts are scarcer, 

 the felspars become prismatic, and biotite increases. Marginal 

 sections show few quartz-phenocrysts, the dominant matrix con- 

 sisting mainly of small felspar-prisms, indicating an approach 

 both in structure and in composition to the Potter-Fell type of 

 intrusive. 



Intrusions of this group are found on Stakeley Tells, Birkbeck 

 Fells, Birkbeck Valley, and the upper part of Stakeley Valley, 

 and one can be traced for 40 yards in a small stream north of 

 the granite. The big dyke on the moor south of Wasdale Beck 

 referred to by Harker & Marr. 2 and the ' sills ' marked on the 

 Geological Survey map in Stakeley Beck and its tributary on the 

 south, are similar both in the held and microscopically. The middle 

 intrusion cuts across the bedding, and as all three are in a line 

 with the nearest edge of the granite, it is probable that they are 

 parts of one big dyke running approximately in a north-north- 

 westerly direction. 



Three dykes exposed more or less imperfectly on the Birkbeck 

 Fells may also be connected. The northernmost is visible as a few 

 scattered patches rising from an expanse of peaty moorland on 

 Hazel How ; the second appears in a rivulet on the south-south- 

 west ; and the third forms a small ridge half a mile farther away 

 to the south-east. The trend varies from north-north-east to 

 north-north-west, and all have a high hade. Idiomorphic quartz 

 and perthite are the largest and most prominent phenocrysts, the 

 middle dyke containing also phenocrysts of micropegmatite. 



1 A. Harker, ' Natural History of Igneous Eocks ' 1909, p. 258. 



2 Q. J. G. S. vol. xlvii (1891) p. 288. 



