FROM THE CROSS FELL INLIER. 519 



been eliminated. These spots consist of crowds of exceedingly fine 

 microlitlis, doubtless of felspar, embedd(>d in quartz, which in each 

 spot behaves as a single crystal. Tlie darker portion of the ground 

 has similar microliths, but in a matrix which remains dark between 

 crossed nicols. The spots, about one-hundredth of ;iii inch in dia- 

 meter, make up most of the rock, which has a superficial resem- 

 blance to certain " spotted slates." In describing a somewhat 

 similar structure in the rhyolites of Penmaenbach *, I was inclined 

 to regard the crystallization of the quartz in the spots as an 

 original character, but the point is not quite clear. 



The rhyolitic rocks exposed in ISwindale Beck show some remark- 

 able features. The dominant type, as seen in the field and in hand- 

 specimens, is a compact pale-salmon or cream-coloured rock, in 

 which darker grey patches, with sharply defined outlines, indicate 

 included fragments. Under the microscope [822] the greater part 

 of the groundmass is obscure, owing to secondary quartz. There is 

 a well-marked, rather wavy parallel structure which might be the 

 lamination of a fine ash, though it is more like the flow-structure or 

 a lava ; and the scattered felspar-crystals, rarely broken, lie with 

 their long axes in the same direction. There are numerous little 

 included fragments of a microlithic andesite, and these would 

 naturally cause the rock to be regarded as an ash, were it not for 

 a special structure well shown in the slide. This is the occurrence 

 of discontinuous bands or narrow streaks, following the flow-lines 

 in the rock, in which a thoroughly crystalline texture is developed. 

 These crystalline streaks consist of clear felspar, often in lath-shaped 

 crystals showing twin-striation, some quartz, and, between the 

 felspars, little areas of pale decomposition-jDroducts, such as usually 

 indicate vanished augite. This Swindale Beck rock is, therefore, a 

 " eutaxitic " lava, which has caught 'up fragments of the rocks, 

 andesitic and others, through which it has broken out. A eutaxitic 

 structure, though of rather different type, has been noticed in 

 .certain Caernarvonshire rhyolites {op. cif. pp. 21, 22). 



4. Acid Intrusive Kocks and Transitional Types. 



The acid intrusive rocks of the district are quartz-porphyries, 

 which do not call for much remark. The best known is the so- 

 called "" Dufton granite," which forms a small boss to the west of 

 Duftou Pike. It is a rock of red colour, resembling some of the 

 •" granite-porphyries " in general appearance. Besides abundant 

 red felspars, it shows quartz-grains about a quarter of an inch long, 

 and small flakes of black mica. Scattered through the rock are 

 hexagonal plates of white mica, up to an inch in diameter ; while 

 occasionally is seen a colourless felspar-crystal, perhaps an inch and 

 a half long, with the markedly tabular habit, the glassy lustre, and 

 the longitudinal fissures (following an orthopinacoidal cleavage) of 

 sanidine. These crystals are twinned on the Carlsbad law, and 

 recall similar ones in a dyke on Stakeley Moor, south of the Shap 



* • Bala Vole. Series of Caeniarv.' (1^89) pp. 22, 23. 



