SOME ANCIENT AND MODERN YOLCANIC HOCKS. 401 



doubt that the triclinic felspar observed under the microscope is 

 oligoclase. 



2. Aran Benlynn (fig. 18). — In this specimen very much of the 

 characteristic hazy structure is obscured by the presence of innu- 

 merable grains and crystals of chlorite with their characteristic ver- 

 micular form. There are many felspar crystals decomposed very 

 much, except just round their edges, which present a clear un- 

 altered appearance. Zirkel describes felspar crystals in porphyrite 

 having a similar structure, being filled in part with decomposition- 

 products and in part with foreign substances, their edge alone being 

 clear and unchanged *. 



In polarized light the chlorite shows colour where it is un- 

 altered and crystallized. Some of the felspar is seen to be distinctly 

 triclinic. 



• "When the analyzer is removed and the polarizer rotated, the 

 many chloritic grains show distinct dichroism in groups, and the 

 heart of very many of the felspar crystals exhibits the same, as if 

 these latter had their inner portions removed and the space filled 

 with chlorite. Where the chloritic granules are not so thickly grouped 

 together the base shows the same coloured-breccia structure as that 

 of the last example, when the prisms are- crossed. 



3. Y Foel-ddu. — This compact light-coloured mass of intrusive 

 felstonef presents the same general microscopic appearance as those 

 just described, except that it is permeated in all directions by strings 

 and patches of the yellowish green mineral. There are many 

 small crystals and fragments of felspar crystals, some of which are 

 certainly triclinic. Thin felsitic veins run through the base in places. 



4. Efridd Trawscoed. — The very compact felstone of this part, 

 the equivalent of that of the Arans, repeats in its microscopic struc- 

 ture the characters already given for the Aran rock. It is traversed 

 in some parts by a very great quantity of yellowish and brown 

 mineral in lines and patches, and in those parts examined presents 

 fewer felspar crystals of any size. 



b. Snowdon Felstones. 



5. Llanberis Route (fig. 19). — The microscopic appearance of the 

 upper part of the Snowdon felstone, on this route, is very similar 

 indeed to that of the Aran Mowddwy rock, presenting quite the 

 same hazy base with a plentiful sprinkling of small particles upon 

 it, and traversed in parts by lines of yellowish and brown mineral 

 (fig. 19). Every here and there quartz may be distinguished 

 from the surrounding felsitic base, and more so perhaps than in 

 the case of the Aran rock. Here also the fragments of felspar 

 crystals consist both of orthoclase and plagioclase. 



6. Glaslyn, Snoivdon. — The top of the same felstone-bed, upon 

 the other side of Snowdon, just above Glaslyn, where it is much 

 cleaved, shows the same felsitic microscopic structure as on the 



* Mikroskopiscke Beschaffenheit, p. 404. 



t Some of the published maps have been wrongly coloured, as if this rock 

 ■were a contemporaneous trap, like that of Aran Benlvnn and Mowddwy. 



2 E 2 



