74 DR. A. K. DWERRYHOTJSE ON INTRUSIVE ROCKS [Feb. I909, 



The Dykes on Yewbarrow and High Fell. 



Specimen 201, collected from a dyke on the left bank of 

 Over Beck, between Overbeck Bridge and Bowerdale Farm, consists 

 of a dark-grey fine-grained rock, much jointed, and presenting 

 the appearance of a rhyolite. The ground-mass is felsitic, with 

 marked flow-structure, and is considerably stained by haematite. 

 There are phenocrysts of felspar, but they are so much decomposed 

 as to render it impossible to identify them with certainty : their 

 habit, however, suggests that of andesine. The rock also contains 

 pseudomorphs after pyroxene or amphibole, or after both. 



A little higher up the stream the dyke is shifted about a sixth of 

 a mile to the north-east by a fault, and then follows the course of 

 Over Beck to the junction of Brimfull Beck with that stream. 

 Specimens collected at 203, 204, 205, 206, & 207 along the 

 course of the dyke, were found to be similar in constitution to 

 that just described. 



Specimen 208, from a dyke crossing Brimfull Beck, is of 

 similar character. 



Specimen 209, from another dyke a little higher up the same 

 stream, is a pink felsite which is non-porphyritic. It consists of a 

 fine mosaic of felspar and quartz, with small wisps of chlorite, 

 probably after biotite. 



Specimen 210, from the dyke which forms the waterfall in 

 Nether Beck, about 100 yards above Netherbeck Bridge, is very 

 similar to 209. 



Specimens 211 to 214, from the dykes so numbered on 

 my MS. map, consist of a highly-decomposed pink felsite with 

 marked spberulitic structure, and containing much secondary 

 epidote and chlorite. 



Specimens 215 to 224 form a network of dykes on the high 

 ground between Nether Beck and Over Beck. They possess the 

 same general characters as Nos. 211 to 214 just described, but 

 vary somewhat as regards the coarseness of their texture. 



Specimens 228 to 231 are of pink felsite, with spherulitic 

 structure similar to that exhibited by Nos. 215 to 224. 



The above form a group of dykes closely related to each other, 

 and possessing marked similarity of composition. 



The Dykes near Allen Crags and Angle Tarn. 



In this group are included Nos. 237 to 242, which are all 

 andesitic in composition, and bear a general similarity both to 

 one another and to the andesitic lavas of the Borrowdale Series, 

 with which they are associated. Specimen 237 will serve as an 

 example. 



Specimen 237, collected close to the edge of a small tarn 



