478 EEV. J. F. BLAKE ON THE 



Fig. 1. — Section in Gorge below Mynydd Hoscolyn, 



Slope of beach. 



certain beds in ^hich there are zigzag lines of quartz dying out on 

 either side and lying obliquely to the bedding. These are referred 

 to by Sir A. Eamsay as " foliated interlaminations of quartz and 

 quartzose schist " ; and Dr. Callaway appears to quote this when 

 he says "there is evidence from the folding of quartz-veins that 

 the rock has been squeezed to the fourth of its original bulk." I do 

 not know that we have any evidence that these were ever any 

 straighter than they are. I interpret them as cracks produced by 

 the irregular oscillation of the overlying quartzite on the more 

 yielding rocks below, and a subsequent infilling by silica. 



As seen in the promontory of Maen-yr-esgyll, the quartzite must 

 be 80 feet in thickness ; it here has a iST.W. dip of 45°, and is cleaved 

 in the same direction at 60°. I give these details to show that we 

 are now dealing with rocks in which such points can be determined. 

 80 feet more of mixed beds lie above it before we come to a 

 second white quartzite. The series is repeated again on the other 

 side of the fault which runs out at Berth Saint, and the upper 

 quartzite is succeeded by grey and yellow cleaved sandstones, and so 

 the series comes to an end against the main fault at Bwa-du. These 

 quartzites, of which the upper may reach to more than 100 feet, 

 are thus proved, stratigraphically, to have no connexion with that of 

 Holyhead, but to be developments higher in the series. The undu- 

 lations of these upper rocks and the development of the quartz- 

 cracks in the beds below the quartzite are well seen in Borth Saint 

 (see fig. 2). It is on the base of this quartzite that Dr. Callaway 

 thinks ripple-marks are to be seen, I interpret the ridges seen, both 

 here and elsewhere, where a massive bed overlies a softer one, to the 

 squeezing-out produced by the cleavage-pressure, festooning the 

 harder in the more plastic mass. 



The other area, which includes the South-Stack island itself, has 

 been well described by Sir A. Eamsay, though there are some im- 

 portant points still to notice. As in the Eoscolyn area, the cleavage 

 is for the most part so intense that it is difficult to make out 

 any succession inland on the broad rolling area by the side of Holy- 

 head mountain. West of the lighthouse there seem to be two deve- 

 lopments of quartz, some of which are worked for china-stone ; but 



