372 MESSRS. A. V. JENNINGS AND G. J. WILLIAMS 



among which arc distributed specks of green mica. It is in fact a 

 typical liornftJs *, such as commonlj' marks the junction of large 

 masses of intrusive rock with beds of slaty character. 



The lower slopes of the mountain consist, on the line of the first 

 section, of somewhat similar rock, but with distinctly bedded ap- 

 pearance and unequal weathering. Higher up, definite spots appear, 

 and w^eather out on exposed surfaces ; in some cases the spots are 

 so numerous that the intervening laminae curve over and under them, 

 so that the appearance is like that of a spherulitic lava. 



Towards the north, round Tan-y-grisiau and under Blaenau, the 

 rocks are pale-coloured with green spots and have a remarkable 

 flaggy structure, splitting into thick slabs with smooth faces. Though 

 regarded by Sir A. Kamsay as due to bedding, this would seem to 

 be rather a cleavage-structure, as the slabs dip at the same high 

 angle as the cleavage of the true slates above. Moreover, the bedding 

 is frequently seen in the same masses as the cleavage, making an 

 angle of 15° with it. 



In some parts the spotted flags contain remarkable concretions — 

 spherical bodies reaching a diameter of two or three feet, showing 

 a rough concentric structure on weathering, and largely calcareous 

 in composition. They are well seen at Tai Matthew (between 

 Glan-y-pwll and Tan-y-grisiau) and at Clos-y-graig. 



Examined under the microscope these rocks also appear to consist 

 of crystalline particles, green and white microliths t with magnetite- 

 dust being disseminated throughout. The white microliths lie in all 

 directions and frequently form radiating groups round the magnetite 

 crystals ; they are lath-shaped or acicular, and polarize distinctly 

 in colours. Probably they are developing micas. The green material 

 is scattered through the sections in aggregations of granules, which 

 are almost isotropic, but seem to be of chloritic nature. It is com- 

 monly the absence of the green material in patches that gives rise to 

 the spotted appearance, the dark areas in reflected light being the 

 transparent ones in sections. In the centre of these lighter areas 

 there is, however, almost always a nucleus of the green mineral. 

 The nature of the spots in these rocks is therefore somewhat 

 diff'erent from that of typical "spotted slates," w^here the appearance 

 is due to local aggregation of the darker minerals. 



In spite of the intensity of the metamorphism and the immense 

 thickness of rock afl'ectcd, there is a striking absence of distinct 

 crystals. The brown mica so common in areas of contact-alteration 

 is absent throughout. In some cases, as Prof. Cole pointed out to 

 us, the spots have, however, a tendency to quadrilateral form, and 

 may perhaps be embryo crystals of andalusite ; but there is nothing 

 in any way similar to the rocks of the altered region about 

 Skiddaw. 



* Teall, ' British Petrography,' p. 373 ; Eosenbusch, ' Die Steiger Schiefer,' 

 Abb. z. geol. Specialkarte v. Elsass-Lothr. vol. i. Heft ii., Strassburg (1877). 



t We have used the term * microliths ' on account of the general appear- 

 ance of the rock. Most of them, when observed between crossed nicols, prove 

 to be crystalline, but not all. 



