﻿Vol. 66.] TREMADOC SLATES OE SOUTH-EAST CARNARVONSHIRE. 177 



X. The Older Dolerites. 



Probably the oldest of the intrusive rocks of the district are the 

 sills of ' white trap ' the outcrop of which I have taken to mark the 

 line of separation between the Maentwrog and the Ffestiniog Beds. 

 These were once dolerites of an andesitic type, but are now become 

 aggregates of calcite, chlorite, leucoxene, zoisite, and other secondary 

 •minerals only. They and their feeder dykes, which trend N". 40° E., 

 form the crags of Craig-ddu and Careg-yr-Eryr, the low'roche 

 fmoutonneein the midst of the Ystumllyn marsh, and again appear 

 in the crags which overlook the Portmadoc golf-links, at Morfa- 

 bychan, and Ynys Gyngar. 



Wherever found, the form and the distribution of the sills are 

 partly determined by the folding of the strata among which they lie ; 

 and as they are also universally affected by foliation and distortion 

 due to cleavage, we must conclude that they are of an age inter- 

 mediate between the folding of the Ynyscynhaiarn anticline and 

 the oncoming of the cleavage. 



XI. The Cleavage. 



The modification of the sedimentary rocks of the Ynyscynhaiarn 

 -district, by stresses at some period subsequent to their accumulation, 

 is a circumstance which forces itself upon the attention of the most 

 •casual observer. All the rocks, except the gabbroid dolerites, show 

 evidence of intense compression, and even the most rigid of the grits 

 have lost their porosity. Speaking generally, all the finer-grained 

 rocks have become slates, and by the uniform rearrangement of their 

 particles have suffered distortion, but have maintained the continuity 

 of their bedding. 



Rigid and coarser-grained rocks have behaved differently. Some- 

 times their individual beds have crumpled up separately, as at 

 the Penmorfa flag-quarry (see fig. 5, p. 174). Sometimes, shearing 

 across, the rigid beds have piled themselves up into ' knoll-reef ' 

 len tides — such as the augen of grit in the Portmadoc Flags near 

 Criccieth, or those of andesitic ash above Tan-yr-allt. Sometimes, 

 again, as among the Ffestiniog grauwackes of Moel-y-Gadair and 

 St. Cynhaiarn's Church, where the compression has been very 

 intense, they have buckled or broken into sharply-pitching isoclinal 

 folds. 



Within our district, the most patent effect of the compression 

 has been a complementary elongation of the mineral particles in 

 a, direction at right angles to the stress. All rocks alike and 

 every fossil found show such distortion, and have been drawn out 

 along an axis which varies from N. 5° E. in the west and south to 

 N. 35° E. in the extreme north-east. A normal steeply-dipping 

 slaty cleavage, also ranging N. 5° E., has been developed in the 

 Arenig and Llandeilo slates of the west ; but over the eastern 

 half of the district the slaty rocks show only a less intense cleavage 

 which at Bron-y-foel dips at 35° to E. 10° S. 



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