CAMBRIAN- IX CAERNARVONSHIEE. 



24: 



Fig. 3. — Jmictioii of Bronlhuifd 

 Grit and Pale Slate, Braich 

 Melyn. 



Silurian slate-quarries of Clyu- 

 nog, which are dug to 100 feet 

 below the level of the nearest 

 Cambrian slate. 



Such is the evidence on which 

 it is inferred that the boundary 

 is for the most part a fault, whose 

 throw increases continually to- 

 wards the south, and which thus 

 accounts for the non-occurrence 

 of the Cambrian slates to the 

 south of Cljnnog. Thus the 

 only locality where we can find 

 a natural boundary is in the 

 neighbourhood and to the south- 

 east of Bethesda. 



The best place to see the 

 junction is on the slopes of 

 Braich Mclyn overlooking Bryn 

 Ogwen, where it can be traced 

 step by step. It occurs in 

 several places in one crag of rock, 

 the lower part of which is Pale 

 Slate, the upper grit (fig. 3). The line of 

 junction makes an angle of about Qb° Fig. 4. — Block of Bron- 



a. Pale Slate. 



b. Bronllwyd Grit. 



with the horizon, and the cleavage of the 

 slate is nearly parallel to it. The base of 

 the grit encloses large fragments of the 

 same slate (fig. 4), sometimes only a few 

 iuches across, but in places becoming 

 huge blocks some feet in diameter. It 

 seems impossible to avoid the conclusion 

 that the Pale Slates had been indurated 

 before the deposit of the grit — in fact 

 that they formed the shore-line on which 

 the grit was deposited. These blocks are 

 confined to the base and are quite angu- 

 lar, and they are not produced by fault- 

 brecciation, for they are irregularly dis- 

 persed amongst the quartzose materials 

 of the uncleaved grit. They do not 

 appear to have been noticed, unless they 

 are the ' mud pans ' referred to by Prof. 

 Hughes ^ and considered to be due to 

 deformation in a massive grit and shale. 

 This, however, is certainly not their 

 character on Braich Melyn. This breccia 

 is also exceedingly well seen at Glan-y- 

 gors, south of Bethesda town, where the 

 1 Geol. Mag. for 1881), p. 10. 

 Q. J. G. S. Js^o. 190. 



llwyd Grit, enclosim/ 

 large fragments of 

 Pale Slate. 



[From Braich Melyu. 

 ■i- natural size.] 



