﻿182 „•: ■ MR. W. G, FEAENSIDES ON THE [May I9IO, 



XV". The Age of the Gabbboid Dolerites. 



I may now summarize the evidence as to the age of the gabbroid 

 dolerites of Tremadoc, T Gesail, Pant If an, and Moel-y-gest. 



(1) The gabbroid dolerites have everywhere altered the rocks 

 in contact with them, both above and below. They are, therefore, 

 intrusive, and later than the Ordovician sediments of the country. 



(2) The laccolites of dolerite follow the general strike of the 

 sediments of the country, but do not follow the course of any 

 individual bedding-plane for more than a few yards together. 

 The outcrops do not, however, partake of the discontinuities that 

 affect the rigid members of the sedimentary series with which 

 they are associated. The dolerites have, in fact, metamorphosed 

 sedimentary rocks which had already attained their present dis- 

 continuous arrangement, and are therefore later than the tearing 

 of the Ordovician volcanic rocks into lenticles. 



(3) The dolerites are completely unaffected by any shearing or 

 foliation which has an orientation related to the cleavage of the 

 country, but, as shown by the distortion and jointing of the rocks 

 which they alter, are later than the cleavage. 



(4) The Moel-y-gest dolerite cuts, and is later than, the Llanerch 

 Fault. 



(5) At Tyddyn-dicwm the metamorphic aureole of the Craig-y- 

 .gesail dolerite extends into the crush-belt of the Penmorfa Fault. 

 Within this crush-belt there is no interfaulting of altered and un- 

 altered rock, but the margin of the metamorphic aureole, as in other 

 places, is regular. The Craig-y-gesail dolerite is consequently later 

 than the Penmorfa Fault. 



Probably, therefore, the gabbroid dolerites of Tremadoc are of 

 late Devonian or Carboniferous age. 



XYI. The Pleistocene Accumulations. 



A discussion of the sequence of events which took place in 

 Ynyscynhaiarn between the time of the later dolerites and the 

 Pleistocene would require consideration of an area much larger 

 than the district studied, and will not here be attempted. 



The Pleistocene accumulations also, though interesting, do not 

 locally afford sufficient evidence of their origin, and these, after brief 

 mention of their lithological character and distribution, I shall leave 

 over for another occasion. 



The oldest Pleistocene deposit that I have discovered in the 

 district is the shore-talus or 'head' occurring close under the cliff, 

 and is overstepped by the Boulder-Clay which is banked against 

 the Criccieth Castle rock. This ancient cliff-deposit is seen on 

 both sides of the Castle, and consists of angular unworn rock 

 with scree-like stratification. It rests quite undisturbed upon a 

 wave-cut platform within a foot or two of the present limit of the 

 highest tides. 



