Vol. 56.] 



GEOLOGY OF NORTHERN ANGLESEY. 



235 



rock is full of small inosculating shear-planes hading northward at a 

 smaller angle with the horizon than the dip of the principal cleavage. 

 Along these shear-planes the matrix is discoloured and dragged out, 

 while the pebbles have been stretched into phacoids and their ex- 

 tremities tailed out. An example obtained from the conglomerate 

 north of Porth Wen Works, where these effects are beautifully 

 displayed, shows one of these pebbles which has attained its present 

 shape by differential yielding along several parallel planes of shear, 

 in spite of which it has remained unbroken. 



While the stratigraphy brings out the general west-north-westerly 

 strike of the beds, the cleavage-strike is conspicuously east and west. 

 Locally, as in fig. 1, these strikes are quite transverse to each other. 

 The obliquity of the cleavage to the strike of the folds and faults 

 shows that the former must be of later date than the latter. Its 

 effect is to obscure the structure of the strata, and sometimes to 

 produce even an appearance of unconformity between two perfectly 

 conformable beds. (See fig. 1, from the coast north of Porth Wen 

 Works.) 



Pig. 1. — Part of an horizontal ledge of rock on the coast, on the 

 western side of Porth Wen Bay, about 90 yards north of the 

 northern Purple Conglomerate. 



.-*■«. V 



'c :*\ 





AB = Grittv slate, the fine lines showing cleavage. 

 CD = Pebbly grit. 



[Length from A to B = 2J feet,] 



The above section illustrates the junction of two conformable beds, the 

 appearance of unconformity being due to distortion by pressure. 



Prom the width of its outcrop near Hell's Mouth the thickness of 

 the purple conglomerate is estimated at 180 feet. The overlying 

 beds are more difficult of measurement, owing to cleavage and to the 

 probable occurrence of undetected faults, but a thickness of 500 feet 

 is suggested. 



(2) Llanlliana Hea d. — The purple conglomerate, as before, lies 

 here upon a massive white quartzite, and dips steeply northward. The 

 pale conglomerates and grits follow, faulted against the purple rock, 

 and they are overlain by black-banded, grey, quartzose shales, which 

 are changed locally into a blue quartzite, and contain above Porth 

 Llanlliana casts of crinoid-stems. 



These rocks are arranged in a faulted syncline, north of which the 



