Yol. 65.] THE OEDOVICIAN AND SILUEIAN EOCKS OE CONWAY. 175 



the outcrop of a plane of movement, along which there has been 

 slight thrusting from a southerly direction. 



I have been unable to find any evidence of the existence of the 

 long fault which limits the southern slope of Conway Mountain on 

 the 1-ineh map, although there is a fault with an east-to-west trend 

 extending from the Sychnant Pass to the Penmaenbach Fault, 

 and more faults with similar trends a little farther south. The 

 Penmaenbach Fault is joined on the south by a most important 

 structural fault, which brings various members of the sedimentary 

 groups against those of the Yolcanic Series. 



V. Detailed Description of the Beds. 



(1) Conway Mountain Volcanic Series. 



The Yolcanic Series is very thick in the western part of the area, 

 but thins away rapidly when traced eastwards. This is particularly 

 noticeable with the highest and lowest members of the series, the 

 lowest being only seen at the western end of Conway Mountain, 

 while the Coetmor Ash-Group diminishes steadily in an easterly 

 direction, and on the eastern side of the estuary is only a few feet 

 thick. The middle members of the series are, however, somewhat 

 more persistent, the main Nodular Rhyolite being still found on 

 the far side of the estuary, with representatives of the Upper 

 Brecciated Lavas in a much attenuated form. 



Conway Mountain. 



(A) Lower Banded Rhyolites. — The various members of the 

 Conway Mountain Yolcanic Series are best displayed at the western 

 end of the mountain, near the Penmaenbach Fault (fig. 1, p. 174). 

 The lowest members of the group of Banded Lavas occupy but a 

 small area, and are not well exposed. They consist of greyish banded 

 rhyolites, which acquire a yellowish tinge when weathered : striking 

 north-eastwards, they are soon lost in the direction of Conway 

 Marsh. A short distance above the 200-foot contour-line dark -grey 

 banded rhyolites (?) succeed them, which are now being extensively 

 quarried both for setts and road-metal ; above these are some 

 similar but paler rhyolites (?), the uppermost members of which 

 are somewhat flinty and light green in colour, while the flow- 

 banding is indicated by lines of opaque dust. These rocks are well 

 seen between the 300- and 400-foot contours, and have been 

 quarried both on the mountain and on the road after being slightly 

 shifted by a tear-fault : they also disappear under Conway Marsh. 

 The remainder of the hill, nearly up to the small peak on the 700- 

 foot contour-line, is mainly occupied by a series of the pale-grey 

 rhyolites (?) which weather yellow, and are either granular or 

 platy, but are always clearly banded. There are, however, at least 



