360 ME. p. LAKE AND I'EOF. S. H. REYNOLDS ON [Sept. 1912^ 



into the surroundiDg matrix, sometimes are sharply marked off 

 from it either by the aggregation of particles of a dark, probably 

 chloritic mineral along the border of the nodule, or by a minute band 

 of quartz. Micropoikilitic structure is extremely characteristic of 

 these rocks ; and, although it occurs in both the nodules and the 

 surrounding matrix, it is especially characteristic of the nodules. 



Banded rhyolites. — These resemble the nodular rhy elites in 

 being practically confined to the eastern half of Mynydd-y-Gader. 

 They occupy a larger area in the field than the nodular rhyolites^ 

 and occur in a broad band into which projects the tongue of 

 dolerite (180). Some 400 yards to the west of this tongue they 

 gradually lose their banded character, and pass into ordinary 

 rhyolite. The weathered surface presents a very striking appear- 

 ance, showing a series of white flinty bands ranging from one-half 

 "to 2 inches in thickness and standing out very prominently. 

 (See PI. XXXVIII, fig. 1.) 



The banding is parallel to the general strike of the rocks, that is, 

 west-south-westerly, and the individual bands, although they some- 

 times curve, run approximately parallel one to the other. The bands 

 sometimes retain a uniform thickness for a foot or more, but fre- 

 quently anastomose with one another and swell out into lenticular 

 forms. 



This coarse banding, seen on the weathered surface, is, as a rule, 

 accompanied by a very fine banding, seen on the freshly-broken 

 surface, and often brought into prominence by the development of 

 strings of a chloritic material. Small phenocrysts, chiefly of quartz^ 

 often show up well on the broken surface in the banded rhyolites. 



In polarized light the rocks show cryptocrystalline alternating 

 with microcrystalline or micropoikilitic bands. Phenocrysts, chiefly 

 of quartz, more rarely of felspar, are decidedly commoner than in 

 the nodular rhyolites, but are not so abundant as in the rhyolites 

 that show neither nodules nor banding. 



Chemical characters of the rhyolites. — Two of the 

 rhyolites were analysed for alkalies as well as silica, and proved to 

 contain a high percentage of soda. The resemblance between these 

 Dolgelly rhyolites and those from Skomer described by Mr. Herbert 

 H. Thomas is great, as is shown by the following analyses : — 



No. 179. No. 28. Soda-rhyoliie, near the 



Ehyolite, southern side Isodular rhyolite, near Table, top of cliff, east of 



of Mynydd-y- Gader, the eastern end of the S2:dt (SkumerJ . 



Dolgelly. Mynydd-y- Gader. (Anal. 345, slide E 7768.^ 



SiO, 78-87 SiO, 72-79 SiO, 79*64 



K.,0 0-54 Iv.O 3-37 K^O 0-38 



Na^O ... 4-28 Na.O ... 5-33 Na,0 ... 6-40 



All these analyses were made by Mr. E. G. Raclley. 



The tuffs or ashes. — Tuffs are splendidly developed to the 

 south of Mynydd-y-Gader, and for hundreds of yards present 



