MONIAN SrSTEM OP ROCKS. 473 



faults, letting" in vertical wedges of black Ordovician shales. The 

 most abundant rock here is the marbled slate, though pelitcs are 

 found at the base, and form the mass of the isolated hill, the highest 

 in these parts, and visible from all parts of the island, called the 

 Garn. This is capped by a massive breccia, which at first I took to 

 be an agglomerate of the series; but I am now convinced it is 

 Ordovician, from the character of the rocks of which it is composed, 

 and which seem to pass up on the eastern side of the hill through 

 grits and finer breccias into ordinary shales. 



Wo now pass to the extreme north-west beyond the trough-fault 

 introducing the Ordovician at Ynys-y-fyddlyu. This area is much 

 <jut up, and there are signs of our being near the centre of eruption 

 in the general coarseness of the pelitcs, which become recognizable 

 ashes, in the occurrence of large masses of agglomerate, and in the 

 intrusions of the granite and other rocks. Of this granite, which is 

 said by Sir A. Kamsay to ramify in all directions. Dr. Callaway says 

 it is a granitoid band of the gneiss, and he asserts that the greenish 

 felspathic beds when followed across the strike to the north are seen 

 gradually to change and ultimately to pass into thoroughly foliated 

 gneiss and granitoidite. To this I cannot assent. After a careful 

 search I can only find one band of granite in Pen-bryn-yr-Eglwys 

 itself, and this runs from the summit of the hill towards the " sea- 

 mark," that is, across any supposed strike of the rocks, which, 

 however, are not sufficiently bedded to have any strike at all. This 

 is associated on its eastern side with a hornblendic rock, which rises 

 into the southern summit and is somewhat foliated. Much of the 

 material surrounding these consists of brecciated rocks of similar 

 material, much impregnated with mica or chlorite, which, being 

 arranged in more or less parallelism, may be said to produce a mica- 

 schist, but it is one of quite a different character from the true foliated 

 rocks. I think there can be little question that these rocks are derived 

 from the breaking up of coarsely crystalline igneous rocks, and are not 

 a stage of metamorphism towards granite. Moreover there is really 

 no passage. It may be difficult in some places to draw the exact 

 line of junction, owing to the fracturing of the granite itself and 

 the subsequent metamorphism : but elsewhere in the same mass we 

 can find a vein of clean granite, bounded on both sides with chloritic 

 breccia of a similar rock, and elsewhere a clear line of junction can 

 be traced in the field. The granite is a fairly clean crystalline rock 

 of large elements, and with comparatively little mica ; but in places 

 it changes into a greenish felsite very like that of St. David's, but 

 with very few scattered crystals, and these not of quartz. Although 

 the clean granite is limited to the line described, the whole material 

 of the hill is composed of the breccia, which becomes more micaceous 

 or chloritic as it nears the granite, and gets finer and finer towards 

 the south. A little further in this direction, near Pant-yr-Eglwys, 

 is another intrusive mass running at right angles to the first, asso- 

 ciated with large agglomerates and ashes, shading off into the finest 

 unaltered dust. This mass is not granite, but a true quartz-felsite 

 with large eroded quartz- and other crystals, and with a granophyric 

 ground-mass, and at its margin it becomes pseudoperlitic. 



