256 



rPvOP. C. LLOTD MORrTAN ON THE PEBIDIAN 



red and green tuffs there are many fragments of a red fluidal 

 quartz-andesite. These fragments are sometimes of large size. One, 

 which I measured was four and a half feet in length. They also 

 contain bombs of basic lava, some of which also are of large size, a 

 yard or more in diameter. 



In addition to these fragmental rocks there are. contemporaneous 

 sheets and intrusive masses of basic igneous rock, the olivine-dia- 

 base of Dr. Geikie, dykes and sheets of felsite or quartz-porphyry, 

 andesite or trachyte, and perhaps sheets of quartz-andesite. 



Concerning the subdivision of the series I do not think that the 

 facts to be presently brought forward justify us in making any 

 definite subdivisions or in going further than stating generally that 

 the series consists of alternating beds of tuff of varying colour and 

 basicity, but with prevailing tints of dark green, red and green, and 

 light sea-green. In the upper beds the tuffs of different character 

 alternate more rapidly, there are more rounded pebbles, and a 

 larger and increasing amount of sedimentary material, together 

 with porcellanite bands. Basic lava-flows occur for the most part 

 in the upper beds of the series. I was quite unable to verify Dr. 

 Hicks's five groups. Indeed their existence is negatived by the 

 folding of the district to be now described. 



Great care, much detailed work, and not a little climbing are 

 required to obtain anything like satisfactory evidence of dip and 

 strike in these Pebidian rocks. Moreover a persistent shearage of 

 the rocks along planes dipping at about 60° to the N.W., giving 

 sometimes the appearance of a true dip, is often very troublesome. 

 Careful observation of the rock-exposures foot by foot will, however, 

 disclose the occasional interstratification of finer and coarser 

 material or the alternation of tuffs of different colour, and will thus 

 enable one to obtain reliable data for determining the nature of the 

 folding which the beds have undergone. 



Detailed work between the headlands of Pen-dal-aderyn and 

 Pen-y-foel thus brings to light the fact that there is, in place of 

 the continuous sequence advocated by Dr, Hicks, or the isoclinal 

 fold suggested by Dr. Geilde, a well-marked syncline, the beds on 

 the southern, side of which are, however, repeated by small faults. 

 The syncline is perhaps tilted inland, so as to form the lip or spout 

 of an elongated basin. 



Purther inland there is again good evidence for the existence of 

 this syncline, satisfactory southerly dips of the northern limb being 

 observed on the prominent craggy exposure south-west of Treginnis 

 Isaf farm. And I t'hink it not improbable that the low-lying swampy 

 ground between Trefeithan and Clegyr Foia marks the continuation 

 of the same synclinal fold*. 



North-west of this central syncline runs an anticlinal fold. The 



^ All along the south-east side of Trefeithan, near its base, the red and green 

 " diabase " tuffs which underlie the igneous sheets are well exposed, and are 

 found to be lying nearly horizontalh', the dip being N.W. (about 10°) in one 

 exposure and S.E. (about 5°) in another exposure at the south-eastern end of 

 the hill. This looks as if we were near the soaiewhat flattened summit of the 

 northern anticline, and that between Trefeithan and Clegyr Foia there runs a 



