﻿Vol. 66.~] TEEMADOC SLATES OE SOUTH-EAST CAEN AE VON SHIRE. 179 



a small syncline on the eastern flank of the Ynyscynhaiarn dome. 

 The course of the fault is sufficiently indicated by the map 

 (PI. XVII), and as a line of topographic features between Moel-y- 

 gest and Ynys-gyngar Parm is very evident. It is traversed by, 

 and is, therefore, probably earlier than, the doleritic intrusion of 

 Moel-y-gest. 



A parallel fault through the hollow along which the road from 

 Portmadoc to Borth Harbour passes is of less importance. It also 

 has a downthrow to the east. 



The faulting which displaces the Dolgelly Beds south of Wern is 

 also interesting ; but, although the fault-face is evident in the road- 

 cutting west of Wern gate, I cannot determine its importance. 



XIII. The Later Doleeites. 



Most prominent of all the topographical features of Ynyscynhaiarn 

 are the great dolerite cliffs of Moel-y-gest, Y Gesail, Tremadoc, 

 and Tan-yr-aJ.lt. These are the most recent rocks (other than 

 Pleistocene accumulations) that occur in the district. They and 

 their attendant metamorphic effects have been described by Dr. Teall 1 

 and again by Mr. Harker, 2 and I shall not at present add to these 

 descriptions. Mr. Harker, however, says that l one effect of the 

 alteration has been to prevent the impression of the cleavage- 

 structure on the argillaceous strata'; and, as he goes on to dis- 

 tinguish this from the examples where cleavage has been obliterated 

 by metamorphism, and so infers that the dolerites are older than 

 the cleavage, I must put forward some additional evidence as to 

 their age. Pirst of all, however, it will be convenient to say 

 something of the jointing of the region. 



XIV. The Jointing. 



The joints in this district are complex, even for a region of older 

 Palaeozoic rocks, but in the plotting of the map their detailed 

 a nations have proved of great value in distinguishing between the 

 ■different rock-series. 



As might be expected from cracks which have opened along the 

 •directions of least breaking strain, during the folding or weathering 

 of the rocks, many of the joints follow the structures of the rocks 

 with great closeness. The direction of greatest elongation of the 

 rock-particles by the cleavage is the direction of the most regular 

 of the joints, which in most rock-exposures serve as ' leaders.' In 

 rock in which a slaty cleavage has fully developed these leaders 

 dip with that cleavage, but in the harder rocks where fracture 

 still in the main follows the bedding-planes, they break at right 



1 • British Petrography ' 1888, p. 216. 



2 'Bala Volcanic Series of Caernarvonshire ' (Sedgwick Prize Essay) 1889, 

 p. 79. 



