^^^' 57'] ^^^ GEOLOGY OF MTNTDD-Y-GAKN. 23' 



and against green breccias on the south. This fault is readily 

 traceable on the ground ; and though its plane cannot actually be^ 

 seen, it is most probably a thrust-fault, on account of the intense 

 contortions of the green phyllites, and because the green breccias- 

 appear to be inverted and to pass below the phyllites. 



t 



Gam Conglomerate^, Grit, and Breccia. 



This formation is perhaps 400 feet thick : its base rests upon the- 

 phyllites, and it passes upward into Black Slates. Its main 

 outcrop crosses the summit of the hill, and forms a band divided 

 into two portions by a transverse fault (the Garn-Castell Fault). 



In addition, on the north-west, near Pen-mynydd, its upper 

 portion is seen, caught in between faults ; and at the opposite 

 extremity of the hill, near Cefn-du-mawr, its upper zones are once 

 more exposed, represented by green breccias. 



The formation is sometimes a coarse angular breccia, sometimes 

 a conglomerate with pebbles fairly well rounded, often a grit with 

 or without pebbles. In its north-western part the fragments are 

 commonly of quartz and grit, and of quartzose, schistose, gneissose. 

 and granitic rocks. To the south-east, towards Cae-engan, pieces 

 of green phyllite are ver}' abundant. Rocks corresponding to the 

 former are found in situ near Mynachdy, 1| miles to the north, and 

 at Pen Bryn-yr-Eglwys, 2 miles to the north-west ; while th& 

 pebbles of green phyllite agree precisely with the rocks of the 

 inlier, and show conclusively that the latter were in their present 

 condition of alteration before the deposition of the conglomerate. 



The Garn Conglomerate has approximately the same strike and 

 dip as the underlying phyllites, and it therefore presents in the 

 field a false appearance of conformity with them ; an illusion 

 which is only dispelled by the evidence of the contained pebbles 

 mentioned above. The conglomerate is often crushed and coarsely 

 cleaved, and its bedding is sometimes obscure. Its dip is northerly 

 and north-easterly, at angles varying from 20° to 75'^ or 80°. Its 

 highest beds contain some layers of black shale, and a transition- 

 zone of black shales with courses of grit and breccia intervenes 

 between it and the Black Slates. 



The green breccias at the southern end of the hill which are 

 faulted against or overthrust by the green phyllites, represent the 

 uppermost part of the conglomerate. They dip towards the 

 phyllites, and contain bands of black shale. They are made up 

 almost wholly of angular pieces of green phyllite,^ and in places 

 are almost indistinguishable from brecciated portions of the phyllites 

 themselves. These breccias are thought to be inverted because they 

 apparently pass down into black shale, whereas the sequence on 

 the east side of the hill shows that the black shales lie above the 

 breccias. 



^ See Quart. Journ. Geol. Soc. vol. Iv (1899) p. 676 : note on microscope- 

 slide N.A. 53. 



