386 EEPORT— 1888. 



Besides those already quoted there are admirable examples of this 

 stracture in the boss of rock in the field on the east of Gaerwen Wind- 

 mill (158), and at the northern extremity of the mass marked ' gneiss ' 

 on the survey map (160) ; in both cases all regularity of stracture on the 

 large scale has been destroyed. The rock at Y-graig (159), near Gaerwen, 

 seems to have been subjected to still greater pressure, since, accompanying 

 an unusual amount of spectral polarisation in the elements, the structure 

 is commencing to show itself on their edges, which break up into optical 

 fragments. A similar explanation applies to a fine-grained rock at Gors 

 Llwyd, Llaniestyn (161), the edges of whose elements are optically very 

 intricate indeed. 



The results of pressure which affect the relations of elements between 

 themselves are various. The simplest is the production of ordinary 

 cracks, which in no way interfere with the remainder of the rock but are 

 filled with new elements, usually of quartz of larger size, but occasionally 

 of felspar, as in the gneiss of Gwalchmai (69). In some cases in the 

 Western district these cracks are filled with a complex group of quartz 

 and green mica crystals, the latter arranged quincuncially, so that the 

 result is very like a fine schist, and in fact illustrates the formation of such, 

 and may be taken for it in fragments as at Llanfechell. There are very 

 few rocks in the whole sedimentary series which are not affected by later 

 cracks, often of two or even more periods of formation, but they do not 

 materially alter the character of the rock. Particular instances, however, 

 have already been noticed in which such cracks are numerous and parallel, 

 and are filled with some flaky mineral, and in these cases they give a 

 banded appearance to the whole mas.", and cause it to split or cleave into 

 fairly thin plates. In the fine-grained rock at Caer Ceiliog (9) and 

 others it has also been noticed that the minute flakes of green mica are 

 arranged obliquely in reference to a series of small parallel cracks trans- 

 verse to the original bedding. The most remarkable example, however, 

 of this strain, produced on the neighbouring material by the formation of 

 cracks, is seen in a rock from the South Stack Series at Porth-y-crug (62) 

 (see fig. 16). The section contains part of the gritty filling of a worm- 

 track, as it seems, in the midst of very fine detritus which has now become 

 silky with the numerous sericite flakes. Possibly this tube of grit has 

 formed a buttress in the movement of the rock, for now the remainder is 

 broken up into a series of cracks, associated in parallel groups, which have 

 had the effect of contorting the fibres into beautiful sigmoid folds, which 

 often now run perpendicular to their original direction. It is a sort of 

 ' strain-slip cleavage ' on a small scale. The whole phenomenon dies 

 away at a little distance from the tube. 



When the fracture of the rock is carried further, the original struc- 

 ture may be quite obscured, all regularity may be lost, and the bulk 

 of the material may be the infilling of the cracks. Such results are 

 by no means uncommon, and, indeed, most of the rocks which do not 

 reveal their nature by a microscopic examination owe their peculiarities 

 to the fracturing they have undergone. The most instructive examples, 

 however, of this structure are met with in the Eastern and Central 

 districts, where the general metamorphism has been the greatest. Thus 

 the rock at Minffordd waste (162), which weathers into pencil-shaped 

 pieces, is more than half occupied by an infilling of chlorite — the frag- 

 ments being of a coarsely crystalline irregular aggregate. So in the rock 

 at Gallows Point, Beaumaris (197) there is so much quartzose calcitic 



