518 DR. C. A. MATLEY ON THE [Oct. I9I3,. 



that it is the same sill as the western band : for even the 

 sedimentary rocks of the island vary remarkably in thickness 

 (as will be shown later), when followed for very short distances 

 along their strike — owing to deformation by earth-movement. This 

 eastern sill is intersected by veins of coarser and paler granite and 

 by lenticular quartz-veins, and becomes finer in texture towards 

 the margins. It is sheared, and shows, under the microscope, 

 severe crushing and advanced ' mortar-structure.' When all the 

 foregoing facts are taken into consideration, the most reasonable 

 explanation of the geological structure seems to be that the sill 

 and its associates have been folded together. 



The quartzite-and-limestone zone mentioned above is cut out 

 eastwards by a thrust, which introduces green white-crusted slates 

 without limestone; in places these slates are cataclastic : they con- 

 tain many lenticles and broken bands of grit and quartzite, and in 

 part they are so sandy as to become cleaved fine-grained sandstones. 

 Under them comes an interesting igneous rock, which can be traced 

 from the shore inland for about 150 yards. It is a crushed, purple- 

 and-green, compact, basic rock, some 6 to 10 feet thick, made up of 

 phacoids, and showing under the microscope variolitic structure in 

 places. It may be a crushed pillow-lava like those described by 

 Miss Kaisin 1 from the adjacent mainland, or possibly a sill. As 

 there is some shearing at the base of this variolite, a thrust may 

 separate it from the underlying rocks, which consist of broken beds- 

 of quartzite and grit and an occasional limestone-nodule in a slaty 

 matrix. The latter beds are thrust, at the head of Bau y Nant r 

 over similar cataclastic rocks that form the northern slope of 

 Mynydd Enlli, which, however, contain in addition a broken 

 massive bed of crushed gritty sandstone, seen in two places. On 

 the east is a crush-conglomerate containing pieces of quartzite, 

 grit, and limestone. 



Whereas the cleavage-planes (and the bedding-planes, where 

 seen) of the beds w r est of the Bau-y-Nant thrust dip almost in- 

 variably westwards, those east of the thrust dip almost as consis- 

 tently eastwards at a high angle. The Bau-y-r\ T ant thrust can be 

 traced southwards on the western slope of Mynydd Enlli for at 

 least three-quarters of a mile. 



(b) The Western Coast. (Part of fig. 2 & fig. 3, p. 520.) 



Along the western coast, between the north-western corner and 

 Porth Solfach, the rocks exposed correspond in the main with those 

 just described as occurring on the northern shore. The granite is 

 found associated in the same manner with the evenly-cleaved green 

 sandy slates, with the more irregularly-cleaved phacoidal and cata- 

 clastic slates and grits, and with the zone of thick limestone and 

 quartzite-len tides already described. There is also to be seen, 

 about 100 yards south of Ogof Gwr, a few feet of a rock that seems 



1 Q. J. G. S. vol. xlix (1893) pp. 148 et seqq. 



