120 ME. J. MOKIUSON ON [vol. lxXlV, 



The rocks include a variety of types, ranging from acid felsites 

 to basic lamprophyres — from a granitic facies to a dioritic facies. 

 Porphyritic and non-porphyritic varieties occur sometimes in asso- 

 ciation, but the latter are frequently microporphyritic. The large 

 orthoclase-phenociTsts which give character to the granite are 

 found also in many of the dykes, and a broad distinction can be 

 made according to their presence or absence. Found in abundance 

 in rocks of varying composition near the granite, they decrease in 

 importance Avhen traced outwards from the centre. Distance from 

 the plutonic centre is not, however, the sole factor determining their 

 abundance or even their presence at all, intrusions in which they 

 are wanting being found in juxtaposition with the most con- 

 spicuously porphyritic, even in the neighbourhood of the granite. 

 Dykes containing the large granitic orthoclases, which (to avoid 

 repeated periphrasis) will be designated ' orthoporphyritic, 1 are most 

 abundant near the granite, rarely exceeding a distance of 4 miles 

 and being confined to the belt already mentioned. Within this 

 somewhat limited area are found rocks ranging from quartz- 

 porphyries to lamprophyres. Associated with them but distributed 

 over a much wider area are rocks of a more usual hypabyssal type. 

 For reasons to be given later these two groups are regarded as 

 belonging to different periods of intrusion, the ' orthoporphyritic ' 

 or three-generation rocks being earlier and more intimately associ- 

 ated with the granite than the two-generation rocks. But certain 

 features common to both, and marked similarities in constitution, 

 clearly indicate their origin from a common magma. 



The intrusions as a whole may be generally classified as por- 

 phyries, porphyrites, and lamprophyres ; but it will be more 

 convenient for present purposes to group them as acid intrusives, 

 intermediate intrusives, and lamprophyres. 



IV. The Acid Intrusives. 



Reference has already been made to the remarkably small number 

 of dykes exposed, compared with the large area affected by the 

 igneous activity which followed the intrusion of the granite. 

 Particularly is this noticeable in the case of the acid intrusives, 

 which are comparatively few in number and generally restricted in 

 direction. That they represent a small proportion only of the 

 actual intrusions is quite evident, especially when the large tracts 

 of drift-covered and peaty moorland in the vicinity of the granite 

 are taken into consideration. The members of this group, apart 

 from the felsites of the adjoining area on the east, are confined to 

 the comparatively narrow belt extending about 8 miles to the 

 south-south-east of the giv.nite and for a somewhat shorter distance 

 in the opposite direction. The rocks vary in colour from pale grey 

 to deep red. All are porphyritic, differing mainly in degree : some- 

 times the phenocrysts are so numerous as to give to the rock a 

 coarse, almost holoci^ystalline appearance ; or a predominating com- 

 pact f elsitic matrix may be but sparsely studded with the porphyritic 



