CHAP. IV.] SILTJEIAN PEEIOD. 47 



doubt that they were soon deeply submerged and covered 

 by the Silurian sediments. 



The evidence for the existence of this Hiberno- Cambrian 

 island is not at present very strong, but it furnishes us 

 with a means of accounting for the great thickness of 

 the Silurian deposits on its eastern side, for if we suppose 

 that the tidal currents came from the south-westward, a 

 reference to the map (PI. I.) will show that they would 

 naturally sweep the debris gained from the erosion of its 

 southern and western shores round to the more sheltered 

 side, and that great banks of sand and mud would be 

 likely to accumulate along the eastern shore and beyond 

 the north-eastern extremity ; the sea itself may originally 

 have been of considerable depth, and the continued sub- 

 mergence would prevent it from being entirely silted up. 



There is better evidence for the existence of two smaller 

 tracts of land to the eastward, which may have been sepa- 

 rate islands, or may have been promontories from a larger 

 tract lying to the north-east of them. One of these was 

 formed by the rocks of the Longmynd and Shelve district ; 

 the Llandovery conglomerate wraps round the edges of 

 this tract, which doubtless had a farther extension to the 

 north-east in Silurian times beyond the range of the 

 Wrekin. The coasts of this island were probably rather 

 steep, and the sea, on the eastern side at any rate, was 

 deep, or limestones would not have been formed in such 

 near proximity. 



* The other island was probably rather larger, and was 

 the remnant of that which seems to have existed over the 

 midland area in Ordovician times, but only the higher ridges 

 between Malvern and Charnwood Forest were left, and it 

 is impossible to say whether these remained above water 

 throughout the Silurian period. 



It is probable, indeed, that both this and the Longmynd 

 island were surrounded by coral reefs which were built up 



