ON THE OLDER HOCKS OF ANGLESEY. 399 



relations whether these are apophyses of the more crystalline rock, or 

 dykes within its substance ; on the whole, the former appears more prob- 

 able. They consist of a rather coarse groundmass of felsitic elements 

 with large idiomorphic crystals of quartz, twinned orthoclase, plagio- 

 clase and mica. The latter rock has much more sericite in its finer 

 groundmass. They are thus * microgranites.' The porphyritic elements, 

 or ' insets,' ' are not always simple crystals, but crystalline aggregates of 

 felspar and quartz or mica, so that it is ' glomeroporphyritic' Most of 

 the felspars are speckled with small crystals, and the quartzes show 

 spectral, or even microspectral, polarisation. The obvious connection of 

 these insets with granite seems to show that these are not ordinary quartz 

 porphyries, but special modifications of the granite itself. 



Another important, though small, area of granite exists in the neigh- 

 bourhood of Pen-bryn-yr-Eglwys, in the north-west. The chief pecu- 

 liarity of the rock here is the almost entire absence of any micaceous 

 element, except such secondary mica as has been developed in well- 

 marked cracks. The freshest rock of the district is behind the farm of 

 Monachty (44). In this the quartz and felspar, most of which is plagio- 

 clase, are of equal importance, crystallising simultaneously in large plates, 

 and occasionally inter-growing pegmatitically. The quartz is much 

 cracked, and shows occasionally some spectral polarisation, and a few of 

 the felspars are speckled ; otherwise it is hard to discover any chano-es 

 which have taken place in the rock, which is as good a granite, or rather 

 aplite, as one conld conceive. It only requires the presence of epidote to 

 bring it into close agreement with the rock of Bryn-y-gam, St. Davids. 

 This same rock at its junction with the surrounding sedimentary rocks 

 (43) loses most of its quartz and becomes brecciated. Its enclosing rock 

 is finely granular and sericitic. 



Nearly as clean a rock as the above occurs on the north side of Pen- 

 bryn-yr-Eglwys (40), but much of the quartz is here disintegrated into 

 smaller elements, and there is some apparently original mica. The 

 accessory minerals observed in these are some small needles of apatite, 

 and minute rhombohedral or hexagonal crystals of brilliant red hematite, 

 a few crystals of pyrites, and very rare zircons. All the other examples 

 from this district are merely cataclastic modifications of this. In one 

 obtained from what was supposed to be the junction of the granite with 

 the surrounding rocks (41), there are numerous and closely set, somewhat 

 parallel, cracks now filled with mica, so that the rock is a kind of false 

 mica schist ; but it is pretty certain from this that the true junction is 

 not here seen. Finally, on the northern side of the hill, close to an im- 

 portant fault (42), the slide shows an indiscriminate mixture of granite 

 fragments and dust, in which there is no regularity whatever. These 

 observations show that the neighbouring schistose rocks are derived by 

 dynamic agencies from the granite, and not the granite from them. They 

 also show how far these changes may go without our losing sio'ht of the 

 previous condition and structure of the rock. "With these granites are 

 associated certain felsites, which seem in the field to take its place. One 

 of these near Pant-yr-Eglwys (35) does not diifer particularly from an 



' No satisfactory name for the crystals in a porphyritic rock has yet been pro- 

 posed, and a paraphrase has always to be adopted The name used by the Germans 

 is ' Einsprengling,' and the nearest English term available corresponding to this 

 would be ' inset,' which it is therefore proposed to use instead of porphyritic consti- 

 tuent. 



