N SY.-TI;M 163 



shire, Hereford, and Monmouth. In all probability they have a 

 onsiderable subterranean extension under Central and Southern 

 England, for they appear as far south as Cardiff and the Mendip 

 Hills, and have been reached in borings at Ware in Hertfordshire 

 and at Eochester in Kent. 



In North Lancashire and Westmoreland they again rise to tin- 

 surface in a considerable tract south of that occupied by the 

 Ordovician, extending from the Duddon estuary on the west to 

 Sedburgh and Rathay Bridge on the east, a distance of about 35 

 miles with an average width of 14 miles. 



In the south of Scotland they cover a much larger area, ranging 

 . "inpletely across the Southern Uplands from the Mull of Galloway 

 to the coast of Berwick. There is also an outlying tract in Lanark, 

 and it is probable that they underlie parts of the Central Lowlands, 

 for the same Lanarkian Beds have recently been recognised on the 

 coast of Kincardine. 



In Ireland there are many isolated exposures in the south and 

 west and a large area in the north-east, occupying parts of Down, 

 Armagh, Monaghan, Cavan, and Louth. 



In most parts of the British Isles there is no great break or 

 unconformity between the Silurian and Ordovician, but it so 

 happens that in the typical districts of Shropshire and Radnorshire 

 a strong local discordance and unconformity is found, due evidently 

 to local uplifts which produced more than one break in the 

 sequence. In other areas the sea-floor may have been raised and 

 the depth of water lessened without any part of it being lifted 

 above the surface of the sea and brought into the sphere of erosive 

 agencies. Under such circumstances uplift is indicated only by 

 changes in the character of the sediment and in the component 

 members of the marine fauna. 



1. The Typical Silurian Area 



We shall consider this as including all the more eastern 

 exposures, and especially those which occur between the Welsh 

 borders and the valley of the Severn. It will embrace not only 

 the Wenlock and Ludlow district* of Shropshire, but that of 

 Presteign in Radnor, as well as those of Malvern, Woolhope, and 

 May Hill in Hereford, Usk in Monmouth, and the outcrops in 

 Staffordshire. These were the places where the beds wi-iv tii>t 

 studied by Murchison, and where they present a more calcareous 

 and more richly fossiliferous facies th;m in any other part of Britain. 



Llandovery Beds. In this region the Lower Llaudovery is 

 absent, and the Tarannon shales are very thin, so that the 



