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MR. 0. T. JONES ON THE HAETEELL-VALENTIAN 



[Nov. 1909, 



is nearly north and south, CDEF is the outcrop of any given bed, 

 and the arrows represent the direction of the dip. When the pitch 

 is to the south (that is, from A to B) the limbs of the anticlines con- 

 verge in that direction, and the limbs of the synclines in the opposite 

 direction; and it will be seen from the triangles pqr, p'q'r con- 

 structed on AB, that in each limb there is a component of the dip 

 in the direction of the pitch, that is, pr for the dip ^g, and pV for 

 the dip p'q. Therefore the outcrop of the beds in either limb, 

 on any slope parallel to AB, will have an apparent dip in that 

 direction. If there were no pitch the terraces would be horizontal, 

 while if the pitch were to the north they would incline from B 

 towards A. 



Fk 



17. — Diagram illustrating the effect of pitch on tlie apparent 

 dip of rock-outcrops. 



Further, so long as AB lies anywhere between CD and DE or 

 between DE and EF, there will always be a component of every 

 dip in the direction AB : if the hillsides in the Pont Erwyd 

 district, for instance, range in a direction anywhere between south 

 and south-south-west, the outcrops of the beds on those slopes 

 will always have an apparent dip in a southerly direction. This 

 effect is very evident on the east side of Drosgol, on the slopes of 

 Dinas and Disgwylfa-fawr, south of Dyffryn Castell, on the sides 

 of the hills bordering the Bheidol Valley between Pont Erwyd and 

 Devil's Bridge, and in numerous other instances. The value of 

 the apparent dip is in general different from, and bears no simple 

 relation to, the pitch. It affords a ready means of detecting 

 whether the folds pitch or not, but the actual value of the pitch 

 cannot be deduced from it. 



