36 PALEOZOIC TIME. [CHAP. III. 



volcanic disturbances, followed by similar conditions of 

 sedimentation, as in the case of the Cumbrian district 

 already described (p. 32). A third centre with a similar 

 sequence is found in the Chair of Eildare on the same line 

 of strike, and other tracts of Ordovician rocks occur still 

 further south-west, in the mountains of Clare and Tip- 

 perary. 



In South Ulster we have a continuation of the South 

 Scottish Ordovicians, and the succession, so far as it is 

 known, is similar ; the base is not seen, but certain shales 

 have yielded the graptolites of the Glenkiln group (Llan- 

 deilo), and others contain Bala fossils, while at Pomeroy, 

 in Tyrone, there is a limestone from which many Bala 

 fossils have been obtained. 



In Mayo and Galway there are shales which are believed 

 to be of Llandeilo age, and others which contain the cha- 

 racteristic fossils of the Bala rocks ; they are estimated at 

 several thousand feet, but the stratigraphy of this part of 

 Ireland requires further investigation. 



2. Geographical Restoration. 



5 From the foregoing facts it would appear that the sub- 

 mergence which began in Cambrian times continued, during 

 the formation of the Arenig series, over the whole of the 

 western and northern portions of the British area, but 

 that over a certain space in the centre of the sea which 

 covered England there was an upward movement resulting 

 in the appearance of an island composed of Archaean and 

 Upper Cambrian rocks. In an east and west direction 

 this island seems to have had a length of at least eighty 

 miles, stretching from the Longmynd in Shropshire to 

 Charnwood Forest in Leicestershire and the neighbourhood 

 of Kettering, and possibly even further to the east. Of its 

 northerly extension we can predicate nothing at present ; 



