H. G. Sargent — The Penmaenmawr Intrusions. 19 



a greenish tinge; this in turn passes gradually into (3) a medium- 

 grained, light-coloured rock of speckled appearance owing to the 

 intermixture of felspar and pyroxene phenocrysts, and the presence of 

 a considerable amount of coarse micropegmatite. Especially towards 

 the margin the rock is intersected by accurately parallel, platy 

 divisional planes, in sets running in different directions and often 

 obliterating each other. These divisional planes appear to be due to 

 contraction on cooling. 



There can be no excuse for dealing in detail with the microscopic 

 structure of the Penmaenmawr rock, in view of what has been done 

 by abler penologists in this direction, but a few observations may 

 perhaps be permitted to indicate the relationship of the different 

 varieties to each other. 1 



The essential minerals present are, in their order of formation, 

 bronzite ; plagioclase and augite (these three occur as phenocrysts) ; 

 orthoclase ; quartz. Flakes of biotite also occur, as well as apatite 

 and ilmenite as accessories. A noticeable feature of the thin sections 

 is the progressive increase of quartz from the margin inwards, and 

 this appears to be accompanied by decreasing basicity of the felspar 

 phenocrysts. 



The marginal rock is characterized by the presence of an andesitic 

 ground-mass, consisting of felspar microlites with bronzite and augite 

 in a second generation of minute crystals and grains ; probably 

 a little glass, and occasionally a small amount of quartz, some of 

 which is doubtless of secondary origin. In places the felspar is of 

 the allotriom orphic type. Flow-structure is frequently very marked. 

 Among the phenocrysts, felspars are, on the whole, rather scarce, but 

 bronzite and augite occur more frequently, the latter predominating. 

 The bronzite is often represented here, as throughout the mass, by 

 bastite pseudomorphs. 



In the fine-grained rock the andesitic structure falls off. The 

 ground-mass consists of felspar laths, allotriom orphic felspar, and 

 increasing quartz ; micropegmatite comes on, and in the medium- 

 grained rock it is beautifully developed and generally forms the 

 entire ground-mass. 



Determination of the felspar phenocrysts in the marginal rock is 

 difficult, owing to the extent to which alteration has taken place, and 

 they are often indeterminable. The microlites of the ground-mass 

 may be referred to oligoclase. 



Thin sections prepared from the fine and medium-grained varieties 

 allow of better determination, the felspars in the former being often 

 quite fresh. Sections normal to the albite lamellae show maximum 

 extinction angles of 25° (in one instance of 30°) in the fine-grained 

 rock. Proceeding inwards towards the centre, the extinction angles 

 diminish to a maximum of about 20°. Schaub 2 finds extinction 

 angles on (010) of 30°, and concludes that the species is near 

 bytownite. In the slides examined by the writer, sections on (010) 

 cannot often be observed, but in a slide (No. 308) prepared from the 



1 Twenty-seven thin sections of Penmaenmawr, two of Carregfawr, and five 

 of Dinas have been examined. 



2 Op. cit., p. 96. 



