^^^' 55-] 0^ SOME OF THE ROCKS OP NOETHERN ANGLESEY. 677 



rocks such as might have been derived from the area (N.A. 82, 83, 

 & 84) near Pen Bryn-yr-Eglwys, 2 miles away, where granites, 

 quartzose gneisses, and biotite-gneisses occur in situ. 



(3) The Quartzites and ^ Quartz-knobs.' 



Under the microscope I have not found it possible to draw any 

 line between quartzites and the rocks of Prof. Blake's ' quartz- 

 knobs.' Evidence of rounded quartz-grains can be generally ob- 

 tained in both types of rock, but is not more easily obtainable in the 

 one type than in the other. The amount of growth of secondary 

 quartz on the grains is not very great, and it is difficult to identify 

 because the old quartz is not much fuller of inclusions than the new, 

 and at times the whole rock is traversed by abundant seams of bubbles 

 which pass from one adjacent grain to another. Still, in most of the 

 slides I have found this secondary growth, and when it is well 

 developed the grains interlock in a way which shows that it is only to 

 be expected that the clastic structure would not be always apparent. 

 In all the slides there is a certain amount of interstitial quartz 

 which takes several forms ; it may be granulitic, microcrystalline 

 and interpenetrating, cryptocrystalline, or minutely clastic. In 

 some cases the adjacent well-rounded grains are coated with fringes 

 of bright sericite (N.A. 31, from a ' quartz-knob ' at Llanbadrig Point 

 [Trwyn y Buarth]).^ The grains often exhibit strain, but not often 

 granulitization. Grains of felspar and other minerals are decidedly 

 rare, and the groundmass is usually purely siliceous, but in some 

 cases it is full of sericite, which has a parallel and schistose arrange- 

 ment. Two specimens were cut from a band of shale included in 

 the ' quartz-kcob ' of Craig Wen. One of them was a rather fine- 

 grained quartzose grit, very much crushed, with its grains embedded 

 in a groundmass of sericitic schist ; the other was a sericite-schist, so 

 much crushed as to be brecciated and to have acquired the structure 

 of a mylonite ; lenticles of quartzite are crushed into it. 



(4) The Results of Earth-movement ; 

 Crush- Con glomerates . 



Nearly all the rocks examined have suffered in some way from 

 earth-movement. Either they are cleaved, or they show strain-slip, 

 or they are brecciated and phacoidal ; and the amount of crushing 

 may be roughly measured by the amount of sericite developed in 

 the groundmass, as well as by the shearing, granulitization, and 

 eventual fraying-out of the contained fragments of quartz. Most 

 of the stages towards the formation of crush-breccias and crush- 

 conglomerates can be traced in the slides as well as upon the 

 ground, and the best examples are as marked and unmistakable 

 as those discovered in the Isle of Man by Mr. Lamplugh. As a 



1 See Greenly, Trans. Edin. Geol. See. vol. yii (1897) p. 256. 



