456 



PROF. W. S. BOULTON OX THE 



[Nov. 1904, 



during its movement and consolidation. 1 From the map it will be 

 seen that, allowing for the displacement produced by the fault, 

 this outlying mass of rhyolite is in the line of strike with the upper- 

 most beds of banded rhyolite (43), or possibly the more acid of the 

 tuffs of the Andesite-Group. 



A specimen (4) about 70 yards from the dolerite, of a dark 

 purple-red colour, and showing vesicles and white veins, has the 

 following microscopic characters ; — The slide shows much veining 

 with inrillings of quartz and calcite, and cavities (originally 

 elongated vesicles or spaces occupied by phenocrysts), which are 

 now filled with quartz and calcite, some of the quartz-crystals 

 containing needles of rutile. Patches of secondary ilmenite 

 altering to leucoxene occur ; while, under a high power, the matrix 

 is seen to be cryptocrystalline, with minute needles of felspar and 

 grains of magnetite. Much brown colouring-matter occurs through- 

 out the matrix, but especially around the filled-up cavities and 

 bordering the veins; it consists of minute rhombs of chalybite,. 

 now oxidized to limonite, and in some cases haematite. Pheno- 

 crysts of felspar up to O05 inch in length are plentiful, mostly 

 with Carlsbad twinning, but with occasional albite-lamellation. 

 The abnormal quantity of calcite and oxidized chalybite, together 

 with the presence of ilmenite, clearly points to metasomatic 

 changes brought about in the rhyolite by the proximity of the 

 dolerite, which at one time probably covered the former. (See 

 p. 482.) 



The silica-percentage and specific gravity of the Northern Rhyolite, 

 together with the silica-percentage and specific gravity of the South- 

 Eastern rhyolite are tabulated below." Some pre-Cambrian and 

 Ordovician rhyolites are included in the same table for comparison. 





I. 



II. 



III. 



IV. 



V. 



VI. 



VII. 



VIII. 



Silica-percentage . 

 Specific gravity . . . 



81-93 

 261 



75-78 

 263 



72-18 

 2-62 



72-57 



... 



83-802 



79-72 



... 



78-40 



73-23 



I. Pale-pink, finely-nodular rhyolite, northern end of Pontesford Hill. 

 II. Dark purple-red, compact rhyolite, south-eastern end of Pontesford 

 Hill. 

 III. Devitrified perlitic pitchstone, from the 'Lea-Bock' Quarry. J. A. 



Phillips. Quart. Journ. Geol. Soc. vol. xxxiii (1877) p. 457. 

 IV. Purple quartz-felsite (pre-Cambrian), from Brithdir Farm, near Bangor. 



J. J. H. Teal!, Quart. Journ. Geol. Soc. vol. xxxix (1883) p. 485. 

 V. Felsophyre, from the summit of Aran Mowddwy, containing porphyritic 

 felspar-crystals in a felsitic matrix. John Hughes, Quart. Journ. Geol. 

 Soc. vol. xxxi (1875) p. 400. 



1 Mr. John Parkinson, F.G.S., who has examined this slide, is of opinion 

 that the rock is a rhyolite-tuff. 



2 I am indebted to Miss Maud Lightfoot, B.Sc, late of University College, 

 Cardiff, for the silica-percentages of some of the acid rocks of Pontesford Hill. 



