﻿178 MR. W. G. EEARNSIDES ON THE [May 1910,. 



In working over this district, I have been unable to obtain 

 specimens that can be made to show the characteristic arrange- 

 ment of mineral particles constituting the needle-cleavage, which 

 so frequently affects the Tremadoc and later beds : so, for the present, 

 I will postpone further consideration of the subject. I cannot, 

 however, agree that a needle-cleavage is the result l of the super- 

 position of two cleavages. 



XII. The Faulting. 



After the cleavage came the faulting, which, as I believe, must 

 belong to a continuation or later phase of the same set of earth- 

 movements as those that produced the cleavage. Elongation of 

 deep-seated rocks can never continue indefinitely ; and, as the pro- 

 longation of the postulated N. 5° E. and N. 35° E. cleavage-axis 

 would lead us among the rigid igneous rock-masses of Snowdonia, 

 it is obvious that expansion could not take place far in that direction. 

 Conversely, the rocks between the district here described and Snow- 

 donia which were also compressed would have to expand south- 

 w r estwards ; and, according to my view, it was their riding to the 

 surface which compelled the formation of the Penmorfa Eault and 

 of the pisolitic-iron crush-belt. The strike-faulting in the country 

 to the north-east of the Penmorfa Eault has been already described 

 (pp. 167-75), and I know of no other faulting in that region. 



The belt of country south-w^est of the Penmorfa Eault is affected 

 by several parallel faults, induced by the shearing movement of the 

 over-riding mass. One of these which, from Glan-byl (see PI. XV, 

 section B) to the top of the woods behind Penmorfa Church, cuts, 

 out the whole of the Dolgelly Beds, must be a thrust of some 

 magnitude, but there are hosts of others too small to map. The 

 rocks on the northern slope of Moel-y-gest also are similarly placed 

 in respect of the continuation of the Penmorfa crush beneath the 

 Morfa, and they, too, are cut into detached lenticles along the 

 strike. It was between two of these lenticles that the pisolitic 

 iron-ore was found and worked south of Penamser. 2 The most 

 evident of the strike-faults of Moel-y-gest is one along which the 

 main road passes north of Penamser (see PI. XV, section A) ; 

 it continues through the Cemetery, and forms the hollow in 

 which Penrhyn-llwyd stands. This fault gives rise to a repetition 

 of the Portmadoc Elags. ; but it is more particularly interesting from 

 the circumstance that it cuts off the north-and-south fault which 

 so markedly displaces the Lingula Elags and Tremadoc Beds in 

 the neighbourhood of Llanerch. This Llanerch Eault (see PI. XVI, 

 section D) is a displacement with a steep hade to the west, and 

 about Llanerch has a downthrow to the east which brings the 

 Dictyonema Band against the Lingulella Band of the Efestiniog Beds. 

 It is associated with a local flattening of the dip, and may represent 



1 A. C. Eamsay, Mem. Geol. Surv. vol. iii (1866) p. 235. 



2 Ibid. p. 252. 



