400 KEPORX — 1888. 



ordinary felsite, with insets of eroded quartz and felspar in an indefinitely 

 crystallised groundmass. But in others, as on the east side of Pen-bryn- 

 yr-Eglwys (37), and at Pant-yr-Eglwys (33), proved to be felsites by the 

 occurrence of numerous, though altered, insets, the groundmass is com- 

 posed of definite round elements, all of the same size and substance, and 

 divided by thin lines of sericite. These elements are so much like 

 grains that, except for the insets, the rock under the microscope might 

 be taken for a sedimentary one. 



In the district south of Traeth Dulas the best preserved granite is a 

 beautiful white rock (139), which microscopically appears quite fresh. 

 It is characterised by its glistening white mica, most of which appears to 

 be original. But in spite of its fresh appearance it is much broken up, 

 and fresh mica is developed along the cracks, the spectral polarisation is 

 strong, the quartz is disintegrated, and a confused orientation is intro- 

 duced. It is, therefore, more or less cataclastic. 



The rock at Llyn Trefwll (55) is still more cataclastic, though desti- 

 tute of mica. The debris are very small, and only a few grains of epi- 

 dote break the monotony. The rock from this neighbourhood, examined 

 by Professor Bonney, appears to have been less broken. There are 

 granites also in the group of rocks to be next discussed, but they will be 

 best dealt with in connection with their surroundings. 



2. The Volcanic Group north-east of Parys Mountain. —This is a mar- 

 vellous group of rocks, quite unlike anything else in the island, or even, 

 perhaps, in Britain. On the large scale, there is no regularity about them ; 

 but they seem to become crystalline in lamps and patches with no definite 

 boundaries, but shading off into earthy-looking or sericitic rocks. Neither 

 their stratigraphy nor their structure, however, suggests that the more 

 crystalline portions are ' eyes ' produced by the relative motions of the 

 several parts ; but give more the idea of their being a volcanic complex 

 of fused and clastic materials. They are not all suitable for microscopic 

 examination ; but such as are, though they may be earthy in appearance, 

 turn out to be crystalline in structure, and the irregularity of the mass is 

 mirrored in the irregularity of the elements. They may be divided for 

 description into two groups, according as the component elements are 

 large or small. Those with larger elements have a crystalline appearance 

 in the field, and some may be classed as granites and some as diorites. 



The granitic type is well seen at various places on the shore, north of 

 Perth Lygan (233). It consists of large masses of quartz divided by 

 sutural lines, and equal masses of felspar which are occasionally idio- 

 morphic, but are now so speckled as to be composed of little more than 

 sericite and calcite, while many of them are banded. Between these 

 two, isolated in their midst, and fitting closely against them, are patches 

 of both brown and white mica, which ai-e, therefore, judged to be original. 

 The rock is therefore a typical granite, showing no signs of pressure, 

 and has no schistose associates. Yet it is of the same type as the other 

 granitic rocks of the island, both in the bands of the quartzes and in the 

 alterations of the felspar, and differs chiefly in the absence of accessory 

 minerals. When, however, this granite is broken, as at Pen-yr-AUt (244) 

 garnets are found in it, though in such a state of alteration, as already 

 explained, that little of the original mineral is left. 



Those rocks on the east side of Parys Mountain, in the band that lies 

 to the south of the main development of the group, that may be referred 

 to the granitic type (252), are. like the rocks near Llecheyn-farwy, more 



