124 MR. J. MORRISON OX ["vol. lxxiv, 



Intrusions immediately north of the granite bear some resem- 

 blance to the plutonic mass itself. One dyke of the orthoporphyritic 

 group contains large pink felspars in a matrix of felspar and quartz 

 with a little biotite ; part of the intrusion, however, is finer in 

 texture, even-grained, free from the large orthoclase-felspars, and 

 •consists of a granular aggregate of felspar and quartz with less 

 oligoclase and more quartz than the foregoing. 



The later dykes are characterized by the absence of the large 

 •orthoclase-felspars common to the preceding group ; but they are 

 .always more or less porphyritic, although the phenocrysts never 

 .attain the importance reached in some of the rocks already 

 described. On the whole, they are more acid in composition 

 (judged by their lower density and mineral composition), and are 

 usually of finer texture. In the field a more or less reddish colour 

 is fairly distinctive in the central parts of the area. The ground- 

 mass in section is almost invariably micrographic, ranging from 

 micropegmatitic to true spherulitic. The rocks usually carry 

 porphyritic quartz and felspar measuring up to \ inch in diameter, 

 in a pale-red, compact, felsitic matrix. Microscopically quartz and 

 perthite are the principal phenociysts, with subordinate oligoclase 

 and a marked decrease in biotite compared with the earlier rocks. 

 The ground-mass is often spherulitic ; but the spherulites polarize 

 in sectors, and rarely show a complete black cross. Most of the 

 phenocrysts are surrounded by spherulitic bands of varying width ; 

 but the maximum is reached round the quartz, each grain of which 

 serves as centre for a radiate growth. Small crystals of perthite 

 merge gradually into a similar growth, the fabric of which is 

 entirely felspathic. In some intrusions the matrix tends to be 

 micropegmatitic, instead of microspherulitic ; while another variety 

 of matrix consists of an intergrowth of quartz and orthoelase 

 polarizing in irregular but fairly-uniform patches, the extinction 

 of which is determined by the quartz. When best developed, 

 micropcecilitic structure is produced. All the rocks contain a small 

 amount of apatite and zircon ; but very little original iron is 

 present, though occasionally pyrrhotite and secondary pyrites are 

 found. 



Intrusions of this type occur both north and south of the 

 granite, the extreme variety at a distance of 8 miles. Red Crag- 

 owes its name to a similar dyke, intruded along a line of fault affect- 

 ing the Bannisdale Slates, and forming a ridge along the summit. 

 Numerous exposures occur on the hills to the south, some of which 

 may be connected. Other examples are found in Stakeley Valley, 

 on Grreenside Crag, Breast High, and as far south as Docker Fell. 



Some felsitic dykes nearer the granite must also be assigned to 

 this group. They differ from the preceding in possessing a micro- 

 granular matrix, although in other respects they are similar. Some 

 of them are associated with more basic rocks, and will be referred 

 to again. The pale-grey porphyritic dyke near Wasdale Head 

 contains abundant pyrrhotite, which also occurs as a constituent of 

 a small basic segregation. 



