38H MR. J. V. ELSDEN ON THE AGE OF [Aug. 1 904, 



Other Secondary Minerals. 



An abundance of quartz, epidote, and calcite would be expected 

 to occur in rocks of this character. In the highly-sheared or 

 crushed rocks, as, for example, at Y Bigil, the quartz-grains have 

 almost the appearance of a clastic origin ; but their secondary 

 character is proved by their sharp extinction when rotated between 

 crossed nicols. With regard to epidote, it may be mentioned that 

 Mr. Harker found this mineral to be restricted to the eastern portion 

 of Caernarvonshire. It is not certain, however, that we can regard 

 this mineral as a normal result of the pressure-metamorphism of 

 diabase. As might be expected, also, both quartz and epidote are 

 not confined to the dykes themselves, but have invaded cracks and 

 fissures in the neighbouring rocks. Very beautiful examples of these 

 quartz-epidote veins occur in the neighbourhood of the crush-area. 

 Calcite-eyes are everywhere abundant, and by weathering-out often 

 give the ' greenstones ' quite a vesicular appearance. It does not 

 seem necessary to dwell upon these phenomena, which are a direct 

 result of the mineralogical changes described in the foregoing pages. 



General. 



Summing up the above results, these rocks exhibit very varied 

 effects of dynamic metamorphism. In their least-altered parts the 

 minerals are comparatively unchanged, with the exception of 

 alterations produced by simple weathering. Coming nearer to the 

 crush-area, we find, first of all, the effects of molecular re- 

 arrangement under pressure without movement. Then the influence 

 of shear begins to appear, with mylonitization and re-crystallization ; 

 and lastly the whole rock becomes more or less cataclastic, with 

 partial or complete obliteration of its original structure. It is not 

 generally possible to draw a sharp line of distinction between these 

 different phenomena, but viewed as a whole the results are suffi- 

 ciently characteristic. Moreover, the gradual appearance of these 

 features, as the dykes are traced from the quartz-felsite into the 

 sedimentary strata towards the east, is a proof that the deforming 

 agency operated from an easterly direction. 



IV. Conclusion. 



In view of the phenomena described in the foregoing pages, it does 

 not seem possible to escape from the conclusion that we have in the 

 Llyn-Padarn dykes a result of the deep-seated conditions prevailing 

 during the latest stage of the Bala eruptions. These dykes appear to 

 have been filled with a magma rather more basic than the Bala sills. 

 The mineralogical evidence seems to point to a larger proportion 

 of titanic acid, and to a greater amount of lime and magnesia. The 

 somewhat-remarkable chemical analysis by Dr. Ycelcker, 1 of a rock 



1 Geol. Mag. 1868, p. 125. 



