600 PROF. T. G. BONNEY AND MISS C. A. RAISIN ON [Nov. 1894, 



ments both in grits and in the well-known conglomerate 1 are 

 mainly : — 



(1) Varieties of felsite which would find their counterparts in 



the ' Llyn Padarn ' mass. 



(2) Quartz-grains, often rounded, sometimes corroded. 

 (8) Felspar-crystals or grains, often twinned. 



All these may well be derived from the quartz-felsite by the 

 lake. 



(4) Granitoid rock, and quartz and felspar, apparently from a 



rock of this nature. 



(5) Quartzite which has a rather characteristic structure." 



(6) An altered-looking, often schistose mineral. It varies from 



pale yellowish to green, is slightly dichroic, and seems to 

 give straight extinction. It bears some resemblance to an 

 altered biotite, but cannot be certainly identified. 



The altered mica-like mineral is especially found in two slides 

 (' post-Llanberis '), but a small fragment of a similar variety occurs 

 in one of the finer ' Cambrian ' strata west of Llyn Padarn, and a 

 similar mineral is seen in some of the quartz-felsite slides. 3 Also 

 fragments of a spherulitic felsite occur in both sets, e. g. in a 

 ' Cambrian ' grit from east of Llyn Padarn, and in one called 

 ' post-Llanberis ' from west of the lake ; and a similar character, 

 associated with a perlitic structure, can be identified in a Moel 



1 S. — East of Llyn Padarn, synclinal by slate-railway. Figs. 1 and 2, d. 

 ' Cambrian.' 



T. — East of Llyn Padarn, by slate-railway. Figs. 1 and 2, g, h. ' Cam- 

 brian.' 



U, V, W. — Three slides from east of Llyn Padarn. Figs. 1 and 2, i. 



X, Y. — Two slides from west of Llyn Padarn, road by Tan-y-pant Cottages. 

 Figs. 7 and 6, a. ' Post-Llanberis.' 



Z.— West of Llyn Padarn, by road soutb of Tan-y-pant. Fig. 7. 'Post- 

 Llanberis.' 



«, j8, y. — Three slides from west of Llyn Padarn, hillside above Tan-y-pant, 

 Fig. 7. ' Post-Llanberis.' 



8, e. — Two slides from east of Llyn Padarn by slate-railway. Figs. 1 and 

 2, a. ' Post-Llanberis.' 



2 On examining a specimen from east of Llyn Padarn (from the hillside 

 above the slate-railway) we find that it consists of subangular to parlly rounded 

 quartz-grains, which obviously have been slightly augmented by secondary 

 quartz, though without losing their original outline. Between them, set as it 

 were in the secondary quartz, are flakclets of a colourless mica. A few grpina 

 of felspar have also been present, most of them now replaced by the above- 

 named mica and quartz, but two or three are unchanged (microcline, plagioclase, 

 etc.). In the groundmass are several granules of yellow epidi te, of pyrite ? r 

 hornblende ?, and possibly ruiile, with an opaque whitish decomposition- 

 product, and two or three rather rounded zircons. The rock obv ; ously was once 

 a somewhat felspathic sandstone. It appears to have been converted into a 

 quartzite before the pebble was made, and is rather more altered than is usual 

 with a Palaeozoic quartzite. This also, so far as one can judge from macro- 

 scopic observation, is true of the other pebbles. 



3 Even some granules of haematite appear to have their equivalents in one 

 or two of the ielstones. (Compare Sir A. Geikie, Quart. Journ. Geol. Soc 

 vol. xlvii. 1891, Pres. Ad dr. Proc. p. 99, and T. G. Bonney, Quart. Journ. Geol. 

 Soc. vol. xxxv. 187'.', p. 311.) 



