Vol. 52.] EASTERN CORNER OF ANGLESEY. 629 



plant-remains abound. At their base is a strong conglomerate about 

 20 feet thick, composed of vein-quartz pebbles with many of red 

 jasper and green schist, as well as of limestone with Carboniferous 

 fossils. Another conglomerate, less persistent, occurs higher up in 

 the group. So far as faulting, which near the East Point is very 

 frequent, permits one to judge, these groups of Seiriol and Trwyn-du 

 appear to be on the same horizon (a thin conglomerate near Llangoed 

 is much lower down) ; though perhaps not accurately so, as in 

 certain sections pebbles and sand can be seen to come in on the 

 same horizon as pure limestone. 



Dark shales occur at all horizons, but are generally very thiu. 

 One of the thickest (about 6 feet) contains some small ostracoda. 

 Prof. T. llupert Jones refers them, doubtfully, to a small ovate form, 

 probably Leperditia acuta, adding, however, that if a feature sug- 

 gestive of a ventral rim or margin be really such, he would be 

 unable at present to determine the specimens definitely. 



The thickness of the whole series appears to be about 700 feet. 

 1 Brecciation \ is not uncommon in some of the limestones, quite 

 unconnected with faulting, and appears to be really an early stage 

 of dolomitization, the change proceeding along the cracks. 



The unconformable relation to the underlying Ordovician Shales is 

 well seen at Careg Onen (fig. 1, p. 619). The lowest beds seen — about 

 20 feet from the base — are dark limestone with shaly partings, and 

 there is no sign of a conglomerate. 



Dykes. — Three dykes mapped in the Baron Hill Rocks show a 

 matrix exactly resembling, under the hand-lens, that of the andesitic 

 dykes described by Mr. Harker (Geol. Mag. 1887, p. 409), with which 

 also they agree in trend and general relations. There is no reason 

 to separate them from that group, arguments for whose Carboniferous 

 or post-Carboniferous age have been given by previous writers. A 

 good many have been already mapped in the schists to the west, but 

 their relations — which are interesting — will not be dwelt on in this 

 paper. The dyke noted by Prof. Bonney (I trust, with him, that 

 the ice-worn surfaces will be scrupulously respected) is quite thin, 

 about 2 feet wide. The optical characters of its porphyritic crystals 

 indicate a felspar allied to labradorite. There is also a dyke, about 

 12 feet wide, cutting the lowest boss of ashy grit in the drive. 



VI. Glaciation. 

 (a) Striation. 



Striae are rare, except on the Carboniferous coast-line where the 

 drift has been but recently removed. 



The prevailing direction is S.S.W., or a few degrees W. of S., 

 except in a district lying east of a line drawn from Dinraor Point to 

 Tnvyn Penrhyn, in which there is cross-hatching with a series 

 running S.S.K. Where they can be compared, the 8.S.W. series 

 can be seen to be the later. 



