316 A. GEIKIE OS THE SUPPOSED 



between granite and lava-form rocks. I may observe in connexion 

 with this subject, that while microcrystalline structure appears to 

 run through all the porphyries, spherulites are not always present. 

 In the dyke or elvan of Nun's Chapel, for example, while some 

 portions of the mass are spherulitic, others are merely micro- 

 crystalline. The spherulitic structure, however, appears to be only 

 exceptionally absent. It occurs not only in large el vans, like those 

 of Nun's Chapel and the Church-School quarries, but even in small 

 veins, such as that which traverses the agglomerate on Clegyr Hill 

 and that which cuts across the tuffs near Pen-y-foel. 



To the porphyries a distinctly porphyritic structure is given by 

 the presence in them of abundant macroscopic quartz blebs or 

 crystals. These are sometimes dihexahedral, usually with some- 

 what blunted angles ; but they also assume irregular rounded forms, 

 occasionally enclosing portions of the base. Porphyritic crystals of 

 plagioclase are common in many of the rocks. 



Some portions of the porphyries where these quartzes and felspars 

 do not appear might be classed as felsites on a cursory inspection. 

 But they all possess the microcrystalline ground-mass. They cannot 

 be confounded with the felsites of which fragments occur in the tuffs. 



Traces of fluxion-structure are discernible in the elvan of Nun's 

 Chapel. The shales at that locality have been invaded by intrusive 

 veins and bands of a rock now much decomposed, but which appears 

 to have been a quartz porphyry. It consists of a decayed ground- 

 mass with much diffused brown matter disposed in lines that sweep 

 round the abundant large quartzes. If it was connected with the 

 adjoining elvan it may show a further stage towards the development 

 of a felsitic rock. But though I have had several slices made from 

 my specimens, they show a rock rather too much decayed to warrant 

 any deductions from them until better examples have been procured. 



Proceeding now from their petrographical to their geotectonic 

 characters, I have to remark that the porplryries occur as bosses, 

 elvans, or veins cutting through all horizons of the volcanic group, 

 and in one case apparently, if not actually, reaching the quartz 

 conglomerate. One of the best exposures of this intrusive character 

 may be seen in the cliff below Nun's Chapel, where the elvan, 

 already so often referred to, runs along the face of the cliff through 

 the uppermost zone of the volcanic group. On the whole its 

 direction is parallel with the strike of the beds. That it is not 

 strictly so, but that the porphyry cuts irregularly through the 

 strata, is well shown at many places*. On one conspicuous 

 precipice the porphyry mass has a thickness of from forty to 

 fifty feet, and lies at an angle of 35°, cutting through the strata, 

 which are inclined, with reversed dip to the northward or towards 

 the land, at from 65° to 70°. 



Apparently in connexion with this dyke a network of intrusions of 

 the peculiar decomposed quartz porphyry above referred to may be 

 observed in the shales along the face of the cliff immediately below 

 Nun's Chapel. On the whole the intruded material has forced its 



* Dr. Hicks has figured this intrusive mass (Quart. Journ. Greol. Soc. 

 vol. xxxiii, p. 236). 



