Vol. 68.] PRE-CAMBKIAN AND CAMBRIAN OP PEMBROKESHIRE. 397 



While maintaining an average west-south-westerly direction 

 they pursue a somewhat sinuous course. The main faults tend to 

 give oif frequent branches, many of which have been traced in the 

 older rocks; others probably exist in the surrounding younger 

 rocks, but these we have not attempted to follow. 



These offshoots in many cases, especially along the northern 

 margin, rejoin the main boundary-faults, and so include lenticles 

 which are faulted on every side. The boundary-faults are therefore 

 not simple fractures, but form part of a complicated and anasto- 

 mosing system. 



It is probable that, in certain instances, the boundary of the 

 area surveyed does not coincide with the line of greatest displace- 

 ment, but with an offshoot from it. South of Ehindaston Moun- 

 tain, for instance, the rocks which adjoin the Lower Cambrian 

 deposits are black shales with abundant Ac/nostus, and arc to be 

 referred to the Menevian ; while a little farther south are Arenig 

 rocks separated from the latter by another fault, the downthrow 

 of which must be far greater than that separating the Menevian 

 from the Lower Cambrian. The displacement between the Menevian 

 and the Lower Cambrian may be still smaller if, as is possible, the 

 thick Solva Series is not represented in this district. The main 

 fracture is therefore located some distance to the south of the 

 Lower Cambrian. 



Similarly, on the north side of the area, the rocks M'hich adjoin 

 the boundary appear to be Middle or Upper Cambrian, whereas 

 a short distance to the north are Arenig deposits. It must be 

 remembered, however, that in this region it is possible that 

 members of the Arenig Series transgress rocks of the older 

 formations. 



The westerly extension of the northern boundary beyond the 

 area surveyed is indicated on the Old Series Geological Survey Map 

 (Sheet 40) by the ' Purple Sandstones and Slates'* of Dinas Mawr, 

 south of St. Elvis, and the * Purple Beds ' of the islet Green Scar, 

 or Sger Las, about a mile farther west. The country to the north 

 of these localities is occupied by Upper Cambrian rocks with 

 possibly some Middle Cambrian. It will be noticed that in a 

 westerly direction the boundary of the Hayscastle area approaches 

 the St. David's area, so that the downthrow of the northern 

 boundary-fault appears to diminish westwards. 



Structures of the older rocks. — The older rocks of the 

 area under description are traversed by several faults which have 

 the same general direction as the boundary-faults and, like them, 

 give off branches which enclose faulted lenticles. The principal 

 fault ranges through Hayscastle, Gignog, and Knaveston, and deter- 

 mines a highly conspicuous valley between the two first-named 

 places. At Gignog the fault appears to split up in a complicated 

 manner, and throws down the Carboniferous rocks between two of 

 its branches. 



At the north-eastern extremity of the district near the Cleddau 



