﻿180 ME. W. G. EEAKNSIDES ON THE [May IQIO r 



angles to the bedding. 'Heading' joints, approximately at right 

 angles to the cleavage-direction, are also common. These generally 

 make an angle of 10° to 20° with the vertical, and dip with the 

 cleavage. They cut across the bedding, and open with a ' rippled ' 

 surface, which is very characteristic. The individual ripples on 

 these joints mark the alternations of harder and softer beds. 



Strike-joints, also nearly vertical, are frequent, but are rarely 

 traceable over any considerable distance. They, too, sometimes 

 show rippled surfaces, but they may be clean cut. 



Dip-joints are never common, and when found are usually 

 ' bevels,' the association of which with the direction of dip is mostly 

 accidental. 



Bedding-planes, cleavage-joints, and either strike-joints or head- 

 ings are the planes of discontinuity, which detach the distorted 

 parallelipipeds of rock that litter the quarries and exposures of 

 sedimentary rocks. 



Of minor jointing the most noteworthy is that which follows 

 the needle-cleavage, and splits up the rocks into the rods, needles, 

 or blades, already described. Where needle-cleavage occurs, the 

 weathered rock breaks into fragments, the long axes of which agree 

 with the main cleavage-direction of the country, irrespective of the 

 strike of the rock. Where undisturbed, the rock-pieces lie inclined 

 along the cleavage at an angle which is some few degrees steeper 

 than the bedding, and maintain their inclination over considerable 

 distances irrespective of the dip of the bed. In most rock-beds the 

 transverse section of the individual needles is characteristic. Some- 

 times almost square, with one face steeply inclined and the other 

 nearly flat (rodded structure), they are at other times flattened 

 down to a knife-blade, with the roughest of conchoidal fracture 

 along their faces (bladed structure) ; and all the stages between 

 these extremes can be found, if sought for. Usually, the cross - 

 section is more or less of a parallelogram, which has its longest side 

 dipping eastwards. 



Besides these jointings, which belong to the minute structures 

 and arrangements of the particular rock-beds, there are also 

 certain joints or structure-lines, which cross the Lingula-F\ag and 

 Tremadoc-Slate country irrespective of everything. These, in the 

 district here described, range between 50° and 60° W. of N., and 

 hade like normal faults. Their directions agree fairly well with 

 that of the Penmorfa thrust-plane ; but, as their inclination is almost 

 vertical, I do not think that the directional relationship can be 

 more than accidental. They are especially evident in the bare 

 Pfestiniog-Group country, south and west of Moel-y-gest and north 

 of Pentrefelin. There, crossing the alternating flag and grauwacke 

 outcrops, they determine the faces of the scarps which are formed 

 by the harder beds ; and hence, though each individual crag marks 

 the outcrop of a hard bed, the alignment of the crags on the hillside 

 or on the map is plagioclinal. 1 



1 C. Callaway, Geol. Mag. 1879, p. 216. 



