Vol. 56.] GEOLOGY OE NORTHERN ANGLESEY. 237 



The broken syncline on the south side of the quartzite-ridge is 

 interrupted by a cross- ridge of quartzite. Conglomerate and black 

 shale occur near Is-allt. 



(5) Forth Padrig. The black argillaceous shales, with Glen- 

 kiln graptolites, of Porth Padrig have a little oolitic ironstone at 

 their base. The outcrop is bounded north and south by faults. 



(6) Porth Pridd. (See PI. XIV, Section 3.) The exposures 

 here fix the horizon of the Penterfyn and Porth Padrig beds. South 

 of the Porth, and faulted against the Llanbadrig Series, may be seen 

 a few feet of a conglomeratic grit, which resembles the fossiliferous 

 beds of Ogof Gynfor. This is succeeded, first by about 150 feet of 

 the black-banded grey beds, and then by a black oolitic ironstone 

 identical in character with that seen at Penterfyn and Porth Padrig. 

 The ironstone passes up into 40 feet of soft black argillaceous shale, 

 similar to that which occupies a corresponding position at Porth 

 Padrig. 



All these beds dip at 65° in a direction a little east of north, and 

 the sequence so far is regular, but from this point northward com- 

 plications ensue. Pirst the banded grey beds, now shattered to a 

 breccia, are repeated by a thrust which brings them against the soft 

 black shale. More soft black shales follow, and then the banded 

 beds, the ironstone, and the argillaceous shales are again repeat^, 

 with small complications due to faulting and thrusting. Finally, tt>c 

 succession is terminated by an overthrust of the Llanbadrig Series, 

 the steeply-inclined thrust-plane being broken, however, en echelon 

 by small cross-faults. 



(7) East of Porth Pridd. — This is a narrow strip of the 

 basal conglomerates and grits. They are cleaved, and for the most 

 part pale, but patches of purple appear in places. Hence the 

 reddish-purple colour of the basement-beds (as seen between Porth 

 Llanlliana and Porth Wen) may be inferred to constitute merely a 

 local feature, and to die out with marked rapidity, indeed almost 

 with abruptness, both west and east of the typical area. The 

 northern boundary is an overthrust of the Green Series ; the southern 

 boundary is also Jfaulted along most of its course, but in one place 

 the pebbly grits lie upon white quartzite. I found one specimen of 

 Orthis calligramma var. Carausii in these beds. 



These areas of Llandeilo rock, insignificant as they are in extent, 

 are of some importance from a tectonic point of view, as they reveal 

 the existence of at least four shattered synclines between anticlinal 

 areas of older rocks. They are not, however, simple synclines, but 

 are themselves usually folded and always faulted. Hundreds of feet 

 of the basal beds are proved to be sometimes absent from the latter 

 cause. The northern boundaries of some, and possibly of all these 

 complex synclines are overthrusts, consequently the Northern Com- 

 plex is not merely a folded, but an overfolded area. 



Q. J. G. S. No. 222. s 



