1 62 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



lumps of pumice. The calcareous sandstone on the top of the 

 breccia is crowded with fossils chiefly in the form of empty casts, 

 and the same material, still full of brachiopods, crinoids, corals, &c., 

 tills up the interstices among the blocks down to the bottom of tho 

 breccia, where similar fossiliferous strata underlie it. 



Nowhere has the volcanic history of a portion of Pala3ozoic time 

 been more clearly and eloquently recorded than in this remote line 

 of cliflPs, swept by the gales of the Atlantic. AVe see that the 

 ordinary sedimentation of Upper-Silurian time was quietly pro- 

 ceeding, fine mud and sand being deposited, and enclosing the 

 remains of the marine organisms that swarmed over the sea-bottom, 

 when volcanic eruptions began. First came discharges of fine 

 dust and small stones, which sometimes fell so lightly as not 

 seriously to disturb the fauna on the sea-floor, but at other times 

 followed so rapidly and continuously as to mask the usual sediment 

 and form sheets of tuff and volcanic gravel. Occasionally there 

 would come more paroxysmal explosions, whereby large blocks of 

 lava were hurled forth until they gathered in a thick layer over the 

 bottom. But the life that teemed in the sea, though temporarily 

 destroj^ed or driven out, soon returned. Corals, crinoids, and 

 shells found their way back again, and fine sediment carried 

 their remains with it and filled up the crevices. The ejected 

 volcanic blocks are thus enclosed in a highly fossiliferous matrix. 



A succession of lava-streams, of which the strongly nodular sheet 

 of Clogher Head is the thickest and most conspicuous, mark the 

 culmination of the volcanic energy, and show how at this late part 

 of the Silurian period felsites that reproduce some of the most 

 striking peculiarities of earlier time were once more poured out at 

 the surface. A few more discharges of tuff and the outflow of a 

 greenish flinty felsite brought this series of eruptions to an end, and 

 closed in Britain the long and varied record of older Palaeozoic 

 volcanic activity. 



I am tempted to off'er here, in conclusion, a general summary of 

 the main conclusions as to Yulcanism which the array of facts I 

 have now brought forward seems to warrant. But I have already 

 trespassed too long on your kind indulgence. With your permission, 

 therefore, I will reserve such statement of results until on another 

 occasion I have been enabled to lay before you a review of the 

 remaining periods in the long volcanic history of our country. 



