274 



PROF. J. F. BLAKE ON THE C^MBRIAX 



Again, the strike of the beds, which is nearly parallel to the surface 

 of junction, runs right into the hill (see fig. i). There raust 

 therefore he another great irregularity of junction, throwing the 

 conglomerates to the south-east, between this and the new church, 

 where they occur again, as noted by Prof. Hughes. Then there is 

 the quarry mentioned by that author by the side of a new road at 

 Yscuborwen *, which I have not found. Here, as at Twt Hill, the 



Pig. 4..— Ground-Plan of Twt Hill. 



Quartz Porphyry 8cc 



^-"^^f Field Quarry 

 >^^ Co7ic/Ioinerale 



^ BlacTc Shales 



conglomerate runs obliquely against the porphyry along a line run- 

 ning S.W. to N.E., and the edges of the beds are cut off by a perpen- 

 dicular line. Here the conglomerate is said to dip, as at Twt, to 

 E.S.E. At a disused quarry in a field by T^^gwyn is another expo- 

 sure of conglomerate and grit, here dipping at 45° to the S.S.E., the 

 line of strike leading directly to the Menai Straits, while the por- 

 phyry is seen eight yards away towards the north. 



The surface of junction, therefore, curves about in a most extra- 

 ordinary manner ; and seeing that the porphyry has the aspect of an 

 intrusive rock, these observations would certainly suggest an intru- 

 sion. Nevertheless they could not have led the Survey to their 

 conclusions, since the line is incorrectly drawn on their map. The 

 alternative hypothesis is a rather complicated group of faults. Other 

 considerations must be brought to bear upon the question before we 

 can decide it. 



The most important of these is the nature of the Twt-Hill rock. 

 ^ Quart. Journ. Geol. Soc. vol. xxxt, 1879. 



