14 H. 8. Jecons — The Breidden and Berioyn Rocks. 



ago when I needed one for teaching purposes.^ On examination of 

 a thin section the plagioclase proved to be alhite, idiomorphic and 

 tabular in form, set in a matrix of chloritic decomposition products, 

 doubtless the remains of pyroxene. The ratio of albite to pyroxene 

 must have been about two or three to one. This composition is 

 sufficient to place the rock in the keratophyre group,* but con- 

 firmation was obtained by comparison with a slide of the well- 

 known keratophyre of Hiittenrode, in the Harz, the rocks proving 

 almost identical both in structui'e and composition. The determi- 

 nation of the felsjjar was made by Becke's bright line method, which 

 proved its refractive index to be everywhere slightly lower than 

 that of balsam. 



The igneous rocks of the Breidden Hills have been ably described 

 by Professor W. W. Watts,^ but I am not aware of any later reference 

 to them. He believed the rocks of Moel-y-Golfa to be mostly lavas, 

 and their plagioclases to be probably labradorite, and therefore 

 named them andesites. His failure to recognise albite is to be 

 attributed solely to the want of the refinements of petrographical 

 investigation wliich exist to-day, Becke's bright line method having 

 been known only since 1893. 



The intrusive rocks of the Breiddens, described by Professor Watts 

 as the 'Newer Series' of igneous rocks, and named by him diabases, 

 may prove to be keratophyres when their felspars are closely 

 examined. I have unfortunately no specimens available. Tlie 

 specific gravities quoted by Professor Watts are of little use as 

 a guide owing to the decomposed condition of the rocks. Should 

 they turn out to be dolerites (diabases), we should have another 

 example of the interesting association of keratophyres with dolerites 

 noted by Rosenbusch,* 



The Berwyn Hills. 



Since 1897 I have been from time to time engaged in investigating 

 the igneous rocks of the Berwyn Hills, which lie to the south of the 

 Dee Valley, between Corwen and Llandrillo. The igneous mass 

 occupying the highest stratigraphical position is a series of quartz- 

 keratophyre (soda-rhyolite) tufi's, with a lava of the same composition 

 afc its base. The fact that sections across the tuffs a mile or less 

 apart invariably show different successions of quartz-keratophyre 

 tuffs, the latter being distinguished from one another by colour and 

 slight corresponding differences in composition, points to a large 

 number of centres of eruption as their source. One of these lay 

 near Blaen Llj'nor, as shown by the agglomerate to be seen in the 

 bed of that stream. 



The four great sills marked ' greenstone ' on the Geological 

 Survey Map of the district, together with their associated dykes 



1 The two specimens available are labelled : — 568, Crags N. of Ty-brya Farm, 

 N. of Moel-y-Golfa summit ; 572r?, S. of road betAveen " Plough and Harrow " and 

 Trewern Fann, S. of Moel-y-Golfa. 



- See definition, Rosenbusch, " Elemente der Gesteinslehi-e," 2nd ed., 1901, p. 287. 



3 Q.J.G.S., vol. xli (1885), p. 532. 



« "Eleni. d. Gesteinslehre," 1901, p. 288. 



