ON THE OLDER BOCKS OF ANGLESEY. 387 



vein stuff that the original fine-grained chloritic material itself looks like 

 a vein in the rest. Again, the rock at Bryn Gorsddu, near Newborough 

 (143), was once one of the ordinary coarse mica schists ; but now the 

 intervals between the fragments into which it has been broken are filled 

 ■with a new and abundant growth of secondary mica, which gives the 

 characteristic appearance to hand specimens. In the railway cutting south 

 of Holland Arms (151) one of the rocks has patches of larger elements of 

 quartz and mica confusedly mixed with smaller elements. Naturally, micro- 

 spectral polarisation is common in such rocks, and in some cases is better 

 shown than in the less fractured ones. For instance, at Cefn Du, Gaerwen 

 (157), in the segregation veins of quartz, this and the crape structure are 

 a perfect picture (see fig. 14). In another case in this district, between 

 Garth Ferry and Beaumaris (194), the bulk of the rock is a mosaic of 

 fair-sized elements, but the whole is utterly broken up and irregular; 

 yet there is orientation produced by the various materials that time after 

 time have filled up the cracks. In the Central district such broken rocks 

 occur near a line along which, on stratigraphical grounds, a fault is 

 believed to run — i.e., at Porth-y-ly-wod (84), west of Gwalchmai (70), at 

 Ynys Coed, Coedana (135), and in the area north of Llangwllog (79). 

 These are all broken up grey gneisses. The rock atTnys Coed, Coedana, 

 is particularly instructive, as, the cracks being filled with abundance 

 of chlorite, it has been taken for a ' greenstone.' There is also another 

 rock in this neighbourhood, north of Llangwllog, which has been called 

 a halleflinta (87), which is so utterly broken up and recemented that its 

 origin is quite doubtful. Other rocks of an entirely confused nature, 

 owing to dislocation and infiltration, have been observed near Bodorgan 

 (88) and Llangefni (101). In the former of these, the shifting is made 

 plain by the dislocations of a small vein which crosses several elements 

 of quartz. The whole of these rocks, though not originating as holo- 

 crystalline ones, may be referred to as cataclastic, since they have under- 

 gone the same disintegrating mechanical processes as those which are 

 so called. 



A more important effect of pressure, because more widely developed 

 and of more theoretical interest, is the production of mylonitic lines. 

 These are peculiar features characteristic of the rocks which have been 

 called Mylonites by Professor Lapworth. The most typical of these 

 mylonites occupy the position of the thrust planes which divide up the 

 rocks in the north-west Highlands. An examination of one of these 

 from the Erribol district will give us the characters of such rocks. We 

 find in it abundant microspectral polarisation, as we should expect, but 

 it also contains, in the midst of crystalline matter, a number of lines, 

 which are remarkable for their continuity, and often for their tenuity, 

 and which part asunder on reaching a fragment, to curve round again 

 into contact when the fragment is passed. Many of them are composed 

 of opaque white dust, whose particles are indistinguishably small ; but 

 many others are converted into sericite, which, when it terminates, is 

 frayed out into indefinitely fine ends, like cirrus clouds. These are the 

 mylonitic lines. In the case quoted, their production has been accom- 

 panied by motion of the rock in their direction, combined with great 

 pressure perpendicular to that direction — in other words, they have been 

 produced by shearing. The converse of this may be safely assumed, and 

 when stratigraphical evidence of shearing is absent, these lines in a rock 

 may be taken to prove that it has been subjected to such a stress. »Such 



c c 2 



