26 ME. C. A. MATLET ON THE [Feb. I9OI, 



cleavage, but in many eases they certainly correspond with the 

 bedding. The slates appear to be thrown into a number of steep 

 folds. On the south and south-west they are faulted against older 

 rocks. 



At Craig-y-gwynt (the ' Telegraph Station ' of the old Ordnance 

 Survey map) a compound anticline of hard bluish-grey quartzose 

 grit, whose base is not exposed, appears below the slates and passes 

 up into them. The conclusion seems to be a safe one that this grit 

 represents the top of the Gam Conglomerate and . the passage-beds 

 to the Black Slates ; but it must be admitted that, although it 

 resembles lithologically the top zone of the Conglomerate near 

 Pen-mynydd, it differs strikingly in general character and in the 

 nature of its inclusions from the green breccias of Cefn-du-mawr, 

 now less than | mile distant. It would seem that the angular 

 green phyllite-fragments were distributed over a very limited area 

 of deposition. 



Oolitic Ironstone. 



A fault runs along the western side of Mynydd-y-Garn in a 

 south-easterly direction across the Black Slates. On the western 

 side of this fault, in a field between Nant-bwbach and Uhald, 

 are two exposures of a rock crowded with oolitic and a few 

 pisolitic grains, which closely resembles the Penterfyn oolitic iron- 

 stone of Northern Anglesey.^ 



In the more southerly exposure it is a black oolitic ironstone or 

 ferruginous mudstone, which passes upward into soft blue-black 

 shale and downward into fine grit. About 8 or 9 feet of it 

 contains oolitic grains, and there may be more oolitic rock con- 

 cealed below the grit. The beds dip steadily north-westward at an 

 angle of 58°. 



In the second exposure, 100 yards away to the north, the beds 

 dip in the opposite direction, namely, south-eastward, at 20° to 30°. 

 The oolitic rock is flaggy, and passes upward into grey grits that 

 resemble the bottom beds of the exposure just described. It thus 

 appears that the grit lies between two bands of ironstone.^ 



The zone appears to be not less than 20 feet thick. Un- 

 fortunately its base is not exposed. Black shales dip towards it 

 as if to pass below it, but they may be cut off from it by faulting. 

 The presence of grit suggests that its horizon is at or near the base 

 of the Black Slates, which is also the horizon of the oolitic ironstone 

 of the northern coast of Anglesey ; but if this be so, the rock should 

 also occur on the slopes of Mynydd-5'-Garn in the zone above the 

 Conglomerate, where I have not found it. It is to be hoped that 

 more outcrops of this interesting rock will be discovered, and that 

 its horizon will be definitely settled. 



^ Quart. Journ. Geol. Soc. vol. Ivi (1900) p. 236. 



^ Mr. J. II. Stausbie, B.Sc, of the Birmingham Municipal Technical School^ 

 was good enough to make a rough assay of one of my specimens. He found in 

 it about 28 per cent, of iron. 



