﻿842 Geological Society : — 



very nearly north and south. This lias been denuded away to the 

 west of Usk, and Silurian beds have been exposed, the rocks seen 

 being of Ludlow and Wenlock age. 



In the southern part of the inlier the Silurian rocks are arranged 

 in two anticlinal folds, the axes of which run nearly north and 

 south and dip southwards. These folds are separated by a fault. 

 The western one is named the Coed-y-paen Anticline, the eastern 

 one the Llangibby Anticline. The Old Red Sandstone is believed 

 to rest un conformably on the Ludlow Beds along much of the 

 margin of the Coed-y-paen Anticline, and beneath the Ludlow 

 Beds, which are about 1300 feet thick, come 35 to 40 feet of a 

 Wenlock Limestone, which covers Wenlock Shales : of these latter 

 some 850 feet are seen. It is impossible to separate the Ludlow 

 Beds into an upper and a lower series, owing to the absence of 

 the Aymestry Limestone. They are composed mainly of sandy 

 shales and sandstones above, and of sandy shales with layers of 

 calcareous nodules or of calcareous bands below. 



Dayia navicula is a common fossil up to 240 feet from the 

 top of the Ludlow Shales, and Holopella gregaria and H. obsolela 

 occur only in the ujrperinost beds. 



At their base the Ludlow Beds seem to pass conformably down 

 into the Wenlock Beds, and the Wenlock Limestone is probably 

 not at the summit of the Wenlock Shales. The Wenlock Lime- 

 stone occurs either in irregular layers separated by sandy shales, or 

 in massive beds largely made up of crinoid fragments. Corals are 

 rare in it. 



The Wenlock Shales below the limestone are divisible into an 

 upper portion, which consists of sandy shales and sandstones, and a 

 lower portion composed of mudstones. The Coed-y-paen Anticline 

 has been much affected by pressure, the hard Wenlock Limestone 

 Bed has been fractured in no fewer than twelve places, and portions 

 of it driven in on to the soft underlying shales. 



In the northern part of the area there is much alluvium and 

 drift ; consequently, although no Wenlock Limestone is now to be 

 seen beyond the Wenlock Shales, it is possible that the limestone 

 may occur beneath the drift, as, when last exposed, the Wenlock 

 Shales are seen dipping north-eastwards, and beyond the drift 

 Ludlow Beds are observed near Clytha. The Llangibby Anticline 

 extends as far north as Cwm Dowlais, and shows Ludlow Beds 

 resting upon Wenlock Limestone, the anticline ending against an 

 east-ancl-west fault. North of Cwm Dowlais nothing but Ludlow 

 Beds are seen between the Coed-y-paen Anticline and the Old Red 

 Sandstone, from both of which they are faulted. 



An account of the Ludlow Beds along the western and eastern 

 sides of the inlier is given, and a large amount of evidence with 

 regard to the ages of the rocks at numerous exposures is produced 

 in the form of lists of fossils. 



The fossils have all been named by Dr. F. R. Cowper Reed, 

 who contributes an appendix in which several new species and 

 varieties are described in detail. 



