522 ME. 0. T. JONES OX THE HAKTFELL-VALENTIAN [NoY. I9O9, 



away along the strike (average strike being understood). The 

 rocks at such places would be invariably higher or lower in the 

 sequence, but the same beds may appear over and over again across 

 the strike. 



I lay some stress on this, because it was a lack of recognition of 

 the strong pitch and its results that led the earlier investigators of 

 the district astray : namely, Sedgwick, Eamsay, and Walter Keeping, 

 who all regarded the Plynlimon Grits and those in the neighbour- 

 hood of Cwm Tstwyth, for instance, as one and the same, because 

 they occurred on the same strike. In reality, however, they are 

 separated by a thickness of strata amounting to nearly a mile. 



It will be observed on the map (PI. XXIV) that the place-names 

 after which the main divisions are called lie more or less on north- 

 and-south lines, as, for instance, Xant-y-M6ch, Bryn-glas, Pont 

 Erwyd, and Devil's Bridge ; therefore the direct sequence is obtained 

 by going along the strike, and not across as in unfolded districts. 



It will be noticed that any given boundary has a definite 

 trend, and the noses of the anticlines or synclines lie nearly 

 on a straight line (due allowance being made for the influence of 

 the form of the ground) which makes a large angle with the actual 

 course of the boundary. Further, the 'trend-lines' of different 

 boundaries on one limb of the anticlinorium are nearly parallel, and 

 those of any given boundary on opposite limbs of the anticlinorium 

 meet at a large angle near its apex. These ' trend-lines ' may be 

 regarded as the generalized strike of the beds in the limbs of the 

 anticlinorium, and lines drawn at right angles to them may be 

 regarded as defining the direction of the generalized dip. If the 

 thickness of strata between any two ' trend-lines ' be known from 

 direct observation, this dip can be calculated. Using for this 

 purpose, on the eastern limb the thickness of the Bryn-glas Mud- 

 stones, and on the western limb the combined thicknesses of the 

 Eisteddfa and the Bheidol Groups, the generalized dips work out 

 at about 20° in each limb, that is, the anticlinorium is symmetrical. 

 Prom the directions of the ' trend-lines ' in each limb, and the 

 value of this dip, the direction of the axis and the average 

 pitch of the anticlinorium can be calculated. The former works 

 out to S. 10° W., which agrees nearly with the direction S. 12° W. 

 of the axis of greatest elevation between Xant-y-M6ch and Pont 

 Erwyd. The average pitch as calculated is 11°, which agrees with 

 observation very closely. It is interesting to compare the thickness 

 of the Plynlimon Stage as calculated (from the value of the pitch 

 thus determined and the width of the outcrop in the direction of 

 the main axis), with the value obtained by direct estimation. The 

 calculated thickness is 3200 feet (allowance being made for the 

 form of the ground) ; while the thickness estimated in the Drosgol 

 section and elsewhere lies between 2900 feet and 3400 feet, accord- 

 ing as the minimum or maximum thickness of the Xant-y-M6ch 

 Flags is taken. The agreement is thus remarkably close. These 

 results show also that the average pitch of the anticlinorium agrees 



