E. B. Taivney — WoodwarcUan Laboratory Notes. 549 



The fine-grained rock at tlie immediate junction is seen to be 

 a diabase. The plagioclase prisms are set mostly parallel to the 

 junction face, and are much decomposed ; the augite is entirely so, 

 onlj^ tracts of viridite represent it ; the ground is darkened by the 

 quantity of magnetite grains, while between them is a greenish-grey 

 granular substance, probably the remains of decomposed felspathic 

 substance and chlorite. 



The third variety of rock is a medium-grained diabase with some 

 hornblende, or proterobase as some might call it. I could find no 

 hard jimction between it and the coarse-grained rock, and though 

 such may be hidden among the furze, it seems to me more likely 

 that the one rock passes gradually into the other ; I infer this from 

 the relative position of the two kinds at their outcrops among the 

 vegetation. 



The chief constituents are augite in large crystals inclosing felspar, 

 etc., and plagioclase ; some of the augite is quite fresh, but there 

 are many patches and aggregates of decomposition products, viridite, 

 etc., which must be put down to it ; the plagioclase is also very 

 much decomposed, and was apparently rich in lime from the calca- 

 reous look of its products. There is a little green hornblende 

 with basal sections giving the characteristic cleavage, but it is in a 

 minority. Brown mica is also present connected with ferric stains 

 and other decomposition products : whether the mica is in an in- 

 cipient stage — ^just forming, or whether it is a remnant decomposing 

 — I am not sure. The ferric stains are also accompanied with 

 opaque black oxide (ilmenite?). 



From the minute quantity of hornblende this rock seems quite 

 difi'erent to the coarse-grained, yet it is quite possible that there are 

 intermediate passages at Penyrhiwan. The case would be only 

 similar to that which Dr. B. Eeyer of Gratz describes (as " Schliere ") 

 in massive eruptions. We may mention the case desci'ibed by Prof. 

 Geikie of pikrite passing into diabase at Blackburn near Bathgate 

 (Trans. E. Soc. Edinburgh, vol. xxix. p. 507). 



The Penyrhiwan boss is apparently protruded through the Cam- 

 brian shales, the exact age of these I am not able to give, not having 

 found fossils. 



Gam Boduan. — In 1878 I had the advantage of accompanying Dr. 

 Hicks when he collected the specimens from this hill, and which 

 were determined for him by Prof. Bonney [Quart. Journ. Geol. Soc. 

 vol. XXXV. p. 305], as a quartz-felsite or quartz-porphyrite. A 

 quarry a few hundred yards to the east, but also on the south side of 

 the mountain, visited at that time, yielded us a rock which on exami- 

 nation I found to be a porphyrite. As Dr. Hicks seems to consider 

 these only as dykes, since the hill is included in his Arvonian in the 

 sketch-map [ibid. p. 297], I felt a desire to examine the hill more 

 closely. Accordingly in 1880 I collected from various points round 

 the hill, and also from the summit. 



We will describe first the rock of the quarry a few hundred feet 

 east of the turnpike on the south side of the hill. In the hand specimen 

 it is a black glistening rock with small separate felspar crystals. 



