•652 MR. C. A. MATLEY ON THE [Aug. 1 899, 



In the region north of Cemaes the rocks have mostly been 

 smashed instead of forming contorted folds, but in tbe shaly 

 limestone of Pig y Barcud and elsewhere there are beautiful con- 

 tortions on a small scale. The intensity of the folding may be well 

 .seen in the large limestone-quarry of Trwyn y Pare. This lime- 

 stone exhibits in the mass no definite dip or strike, and bedding can 

 rarely be made out, except where a coal-black crushed band of 

 shale twists about on the south side of the quarry, or where thin black 

 carbonaceous films make the structure evident. The limestone has 

 the appearance of being built up into vertical prisms, and, save for the 

 colour of the rock, the observer might readily imagine that he was 

 looking at a quarry of basalt exhibiting vertical columns and joints. 

 Though the structure of the massive limestone has been largely masked 

 by jointing, etc., two remarkable folds in the quarry show that it has 

 also been intensely folded. These folds face each other on opposite 

 sides of the quarry. That on the north side shows the junction of 

 the limestone with the overlying beds, which are represented by 

 broken shaly material weathered into a muddy mass. The lime- 

 stone has been curled up on every side into a basin-like fold with 

 vertical sides, in shape like the bowl of an egg-cup or wine-glass, 

 which would be better seen if the soft shaly stuff in the interior 

 were scooped out. The axis is vertical and the curving-in of the 

 encircling wall of limestone to form the base of the cup can be 

 ■well seen. The diameter of this fold varies but slightly, say, from 

 4 to about 4| yards. The front has been quarried away, and 

 the upward continuation of the fold has also been removed by 

 quarrying or denudation. The second fold, which is similar but not 

 so regular, is to be seen 30 yards to the south on the opposite side of 

 the quarry, and this too is filled in by a weathered mass of broken 

 yellowish- green and black shaly material with less weathered 

 harder pieces that represent the strata succeeding the limestone. 



Such folds as these cannot be produced by earth-movements 

 acting in any single direction. We must have a combination of 

 forces acting either simultaneously or consecutively in at least two 

 directions, and these directions, if only two, would have to be at 

 right angles to each other. One of the folding forces may be taken 

 as having acted transversely to the north-and-south line that joins the 

 xjentres of these two.folds, that is, in an easterly or westerly direction. 

 This leaves for the second force the movement from the north to 

 which reference has already been made. 



(b) Dislocations. 



Faults form so commonly the junctions between the rock-groups as 

 to make the relationship of the various strata to each other a matter 

 of extreme difficulty. The section at Ogof Gynfor (fig. 3, p. 648) 

 affords a good instance of the terribly fractured nature of even the 

 Ordovician rocks of the district. It exhibits normal faults, reversed 

 faults with a more or less curved fracture, and gently sloping thrust- 

 planes. We have already seen instances of dislocations on a larger 



