654 



ME. C. A. MATLET ON THE 



[Aug. 



section indicates to my mind not ' ordinary faulting resulting .... 

 from tensiou,' but thrust-faulting due to pressure. 



In connexion with the overfolding mentioned as occurring around 

 the shores of Cemlyn Bay, there seems to have been plenty of 

 shearing movement on a small scale at an angle agreeing with or 

 only slightly oblique to the bedding. It is often difficult actually 

 to prove this, although some of the planes are unmistakable. 

 Movement gently oblique to the beds is inferred from the curious 

 ' wedging-out' of individual strata when followed either along the 

 dip or the strike. Individual laminge are seen to terminate along 

 some of these planes ; this pro- 

 Pig. 6. — Forth Neiuydd : small 

 horizontal faults in the cliff north 



■duces an appearance of false 

 or current-bedding, but it is 

 due, I think, to the trunca- 

 tion of the laminae by shear- 

 planes. 



There are other horizontal 

 faults which indicate a general 

 shaking and shattering of the 

 beds rather than a movement 

 in a definite direction. Such 

 are the tiny dislocations al- 

 ready mentioned near the 

 Forth Newydd fault, where 

 hundreds of them are revealed 

 by the banding of some nearly 

 vertical beds. A small por- 

 tion of a rock-face exhibiting 

 twenty-eight of these miniature 

 faults is shown in fig. 6, 



(cj Cleavage. . 



The phenomena of cleavage 

 have not been closely studied, 

 but a few points may be 

 brought forward. Though the 

 rocks have been subjected over 

 the whole of the area to close 

 comi^ression, yet it is only in 



of the Ordovician houndary. 



[Distance between bands=about 1 inch.] 



a few localities that they are seen to split obliquely to the bedding. 

 The black shales of the Ordovician around Carmel Head are sometimes 

 changed to argillaceous slates, and the coarse, black-banded, grey, 

 sandy shales (also Ordovician) of Llanlliana and the neighbourhood 

 are similarly cleaved, but less perfectly. Along these planes quartz- 

 veins frequently appear. Even the conglomerates have been affected, 

 and new structural planes are seen to cut across the bands of 

 pebbles and determine the splitting of the mass. 



The strike of the most evident cleavage is more or less east-and- 

 west, but does not always agree with the strike of the rocks. In 



