116 STKATIGKAPHICAL GEOLOGY 



Eocks of this age occupy a large part of the Lake District, 

 where they were first studied by Sedgwick. The Isle of Man is 

 an isolated fragment of this Cumbrian area, while to the eastward, 

 in Yorkshire, some small exposures occur, proving the extension 

 of Ordovician rocks beneath the newer strata in that direction. 



In the south of Scotland, Ordovician and Silurian rocks range 

 across from Wigtown to Berwickshire, and form the mass of the 

 Southern Highlands, their northern boundary being generally the 

 great line of fault which runs from Girvan in Ayr to the 

 Lammermuir Hills. 



Ordovician rocks occur in many parts of Ireland and exhibit 

 several different facies, one of these facies being found in the north- 

 west (Galway, Mayo, Donegal, and Londonderry), another in the 

 north-east (Down, Cavan, and Meath), another in the south-east 

 (Wicklow, Wexford, and Waterford, with extensions into Clare and 

 Tipperary). 



In most places the Ordovician rocks seem to succeed the 

 Cambrian in conformable sequence, but in North Wales there 

 appears to be a decided break of continuity, the basal grit of the 

 Arenig Series lying unconformably on the Cambrian and passing 

 transgressively across the members of the Tremadoc Series on to 

 those of the Ffestiniog Group. From an examination and survey of 

 the country around the Arenig and Araii Mountains Mr. Fearnsides- 

 has expressed his belief that the Cambrian rocks of North Wales 

 were ridged up into a series of low flexures striking north and 

 south before the deposition of the Arenig sediments. This physical 

 disturbance was probably due to the intense volcanic action which 

 shortly afterwards led to the establishment of surface volcanoes, 

 and to the outpouring of the great masses of lava and ashes which 

 are such conspicuous features in the Ordovician Series of Merioneth 

 and Carnarvonshire. 



1. JFales 



Arenig Series. The typical district of this group is supposed 

 to be that of the Arenig Mountains in Merioneth ; but the series 

 as now restricted is not there very thick, and it will be more 

 convenient to begin with South Wales, Pembroke, and Carmarthen, 

 where it has a fuller development. 



The outcrop of the Arenig Series on Eamsey Island and round 

 St. David's Head is shown on Fig. 16, p. 80. From the latter point 

 its outcrop runs eastward and curves round the St. David's anticline 

 to near Haverfordwest, and thence eastward to Carmarthen and 

 Llangarthney. 



