530 EEV. J. p. BLAKE ON THE 



Llangefni. The principal difficulty is that there is another small 

 quartz-knob near l^ebo, and a more reef-like mass near Caeau Hen, 

 both in the midst of Ordovician, and it might be difficult to give 

 convincing reasons for separating the larger one near Parys Moun- 

 tain from the other two, or to believe that there were three masses 

 of old rock here. Still, this looks like the knobs elsewhere, and the 

 other two do not. 



Summary of the District. 



This district is bounded by faults except between N'ebo and Plas 

 IJcha, where it is only broken and has basal Ordovician apparently 

 between it and the black shales. Eocks resembling grey gneiss 

 may be found on the boundaries on both sides, but the main mass 

 of the district is composed of a volcanic accumulation, or of granitic 

 and dioritic rocks, inextricably mixed with fragments derivable from 

 similar rocks, which have undergone metamorphosis and become 

 mica-schists of a peculiar type. The rocks of Parys Mountain are 

 not in any way connected with the district and are not Pre-Cambrian, 

 but there is a quartz-knob to the south-west which probably belongs 

 to the series. 



Desceiption op Coeeelated Eocks in neighboueing Areas. 



DiSTEicT OP THE Lletn Peninstjla (sco PL XIII. fig. 2). — This is 

 so far removed from Anglesey that no stratigraphical connexions are 

 to be expected, but the lithological resemblance of the rocks in the 

 two areas is so great that by common consent they have always been 

 included in the same series. The Lleyn district is so mach more 

 inaccessible that I have not been able to afford it so detailed an 

 examination as the other districts, of which, however, it is in every 

 way worthy. 



The boundary of the rocks considered Pre-Cambrian is the 

 straight line marked on the Survey map, which there is every 

 reason to believe is a line of fault. This is indicated at the north- 

 ern end by the sudden change in the rocks in the neighbourhood of 

 Nevin, and at the southern end, on the east side of Aberdaron 

 Bay, the fault is actually seen. It has been accurately figured and 

 described by Sir A. Eamsay. 



The main mass of the rocks in this district belongs to the upper or 

 volcanic portion of the series as seen in Anglesey, though largely mixed 

 with rocks of a more slaty but stiU ashy character. Sir A. Eamsay's 

 description of these is very graphic and leaves little to be desired. 

 There is the same development of sporadic limestones and quartz- 

 knobs, with associated purple slates, that we have seen in other 

 districts. The general strike, so far as can be made out, cuts the 

 fault obliquely, and the dip seems to be such that the higher por- 

 tions come on towards the north, but there need not be any very 

 great thickness. The reason of this is, that they are so irregularly 

 stratified that it is difficult to make out any sequence, and the 

 varieties may be related to horizontal and not to vertical position. 

 One of the most schistose portions is in Abergeirch, where one is 



