506 



EEV. J. F. BLAKE O^T THE 



the Llangaffo cutting, and yet they are remarkably clean and per- 

 fectly distinct from their intrusive neighbour. This occurs in many 

 different examples from various localities, most of which are near 

 the margin of the visible diorite. A third and perhaps the most 

 prevalent variety is that in which there is a great development of 

 secondary mica. There may also be difference in constitution in 

 these, since felspar seems to be less abundant, and they may with 

 propriety be called simple mica-schists. In many of these the mica 

 may be seen passing over into chlorite, and in this case the resem- 

 blance to the most altered forms of the chlorite-schists is very 

 close. "When they are greatly disturbed they lose all signs of 

 bedding and become like bundles of pencils, whose points stand out 

 like teeth from the fractured end of the rock. 



Such is the material which, with a remarkable exception to be 

 hereafter described, and with its associate, the diorite, occupies the 

 whole of the district south of the Holyhead road, and from thence 

 on the western side of a line from Llansadwrn to Llaniestyn. On 

 the continuation to the north it rises into the wild rngion of 

 Mynydd Llwyddiart, where it has a far more quartzose aspect, which 

 may in part be due to weathering ; and on the east it forms the 

 ragged ground of lEynydd Crwgarth, where the pencil- variety is 

 most abundant. 



It is on the edge of these rocks from Wern to Minffordd that the 

 Cambrian rocks repose, and on the other side also they form the 

 basis on which the Ordovician rocks repose. These latter consist of 

 grits, well seen both south and north of the Holyhead road. To the 

 N.E. of Llanfihangel Esgeifiog, at the farm of Ehyd-yr-arian, the 

 basal conglomerates are seen, and in the roadside a great thickness 

 of hard brown slates. Sir A. Ramsay records in tbese rocks at Plas 

 Berw several fossils which he assigns to the Bala series. They 

 are the same, however, that occur at Treiorwerth, and the earliest 

 date that can be assigned to them is x^enig. AYe thus have evidence 

 that the Cambrians came no further than the area of grey gneiss, 

 and thus nearly the whole of Anglesey was dry land during the 

 whole of the Cambrian Period. 



The Noetheei^ SuccESsioif. — The upward continuation of the grey 

 gneisses is mostly cut off by the intrusion of the diorite, and where 

 this appears to die away on the north a rather wide valley separates 

 the exposures of rocks. But as one or two spots on the east of the 

 diorite, as Tyn-j^-gors, are still like the grey gneiss in internal 

 structure, though very unlike in external aspect, we may assume 

 that there is a gradual passage. The rocks round Menai Bridge are 

 of that irregular fine-grained type which is seen on the mainland in 

 the western district, and is very distinct from the laminated true 

 chloritic schists. They are therefore but little below the com- 

 mencement of the pyroclastic series, yet it is impossible to draw 

 any real line where one begins and the other ends (though it has to 

 be drawn on the map). South of a line from Dinas to Garth Perry 

 they are most crystalline in structure and may thus be marked off 



