CHAP. III.] ORDOVICIAN PERIOD. 39 



be said of the succeeding restorations of Palaeozoic geo- 

 graphies, so little do we really know of these rocks and 

 their extension beneath the British Islands. 



During the deposition of the Llandeilo and Bala rocks 

 depression continued, and the whole of the British area 

 seems to have been submerged, with the exception of a 

 portion of the central English island ; the dimensions of 

 this island were of course considerably diminished, as 

 testified by the extension of the Caradoc sandstones to the 

 east of the Longmynd, but these do not extend into 

 Staffordshire or Warwickshire. Llandeilo and Bala rocks 

 seem to have been laid down over the whole of Ireland, 

 and if any part of the land barrier above mentioned re- 

 mained above water it must have been outside the present 

 limits of Ireland. 



Numerous volcanic islands, however, came into existence 

 during this period, and portions of the lava-streams which 

 they emitted are interbedded with the sedimentary rocks 

 of Llandeilo and Bala age. In Llandeilo times a line of 

 such islands seems to have stretched across Ireland and the 

 north of England in a south-west and north-east direction, 

 while in Bala times great eruptions took place from a 

 group of volcanoes in Wales and the east of Ireland. 



The period was brought to a close by an upheaval which 

 brought up the submerged ground, but gave it a different 

 outline and extension from that which it possessed in 

 Arenig times. 



