﻿Yol. 64. J GEOLOGICAL STETJCTURE OF THE ST. DAVID's AREA. 379 



sure of this point, this special area has been mapped out on the 

 scale of 25 inches to a mile (fig. 1, p. 378). It was this detailed 

 mapping that led to the discovery of the only known normal 

 junction of the Cambrian and the granophyre, and the fact that 

 this existed would probably have never been known but for the 

 recent establishment of the Gas-works. Before these were erected, 

 there was a farm-service road leading from the high road to a 

 quarry in the conglomerate. This road formerly kept clear of the 

 bank flanking the Alan in which the quarry occurs. In this bank- 

 face, quite at the foot of the hill, fragments of the conglomerate 

 only occurred, and it was a natural inference that this rock con- 

 tinued to the foot of the bank. When the Gas-works were built, 

 the road had to be shifted closer to the bank, and, in so doing, the 

 fault already mentioned as shifting the Cambrian at Ogof Llesugn 

 was cut open, showing that the granophyre occurred on both sides 

 of it. 



It was thus obvious that there must be in the bank-face, on the 

 west side of this fault, the original junction of the granophyre and 

 the Cambrian conglomerate. Mr. G. Barrow, when on a visit to 

 me, drew my attention to the supreme importance of this fact, and 

 suggested a further visit with the object of cutting this junction 

 open. This has since been done, and the section laid open exposes 

 the Cambrian conglomerate resting upon the eroded surface of the 

 granophyre. The position of the small opening made is shown on the 

 25-inch map, and has also been photographed ; it has been left open 

 for verification by subsequent observers. 



The basal band of the Cambrian here is so like the decomposing 

 Dimetian, that in a first opening made it was actually taken for it, 

 and it was only distinguished by some very small pebbles of the 

 characteristic pink quartzite which occurs in much larger pebbles 

 higher up in the conglomerate. The actual junction is found in a 

 second opening : the only means of distinguishing the two rocks at 

 first was the presence of these minute pink pebbles, and it was not 

 until the opening was enlarged that the junctions were defined. 



It is clear that the basal band of the conglomerate is the finest 

 debris of the granophyre, and however we may account for the 

 fineness of this base of the conglomerate, it is accompanied by an 

 equivalent diminution in size of the scattered pink quartzite-pebbles. 

 There is no trace of faulting, or of any marginal modification of the 

 granophyre, and no sign of thermal action is seen in the Cambrian 

 when actually touching the igneous rock. The latter, on the 

 contrary, shows clear signs of decay previous to the deposition of 

 the Cambrian Conglomerate. 



Basic Intrusions. 



The basic igneous rocks of the district have been classified by 

 previous observers into two distinct series — (a) intrusive dykes, and 

 {b) contemporaneous sheets or lavas. It is universally agreed that 

 the former are post-Cambrian ; but the latter have been held to be 



