ANNIVEESAKY ADDKESS OF THE PRESIDENT. 61 



We have seen that the later Silurian ages were marked b}' a 

 diminution and final cessation of volcanic activity v^ithin our 

 borders. The great Arenig, Llandeilo, and Eala eruptions of Wales, 

 of the Lake district, of the south of Scotland, and of the east of 

 Ireland had long passed away : their cones had been washed down, 

 and their sheets of lava and tuff had sunk under a thick pile of over- 

 lying sediments in which no trace has been found of any contempo- 

 raneous volcanic eruptions. In the far west of Ireland, however, 

 remains of volcanic outbreaks appear among Upper Silurian rocks, 

 but their limited extent and thickness sufficiently mark the feeble 

 condition into which volcanic action had now passed. Nevertheless, 

 other evidence is not wanting that terrestrial disturbances affected 

 our area during the later ages of the Silurian period. The bands of 

 coarse conglomerate, which form so notable a feature among the 

 Upper Silurian rocks, may indicate such disturbances, even when no 

 actual unconformability can be traced between them and the strata 

 on which they rest. In this connexion it is not unimportant to 

 observe that in the west of Mayo, Galway, and Kerry, where the 

 Upper Silurian rocks include vast mountainous masses of conglo- 

 merate, and where therefore terrestrial instability may be supposed 

 to have been greatest, the few recognizable traces of Upper Silurian 

 volcanoes are to be found. 



There can be no doubt, however, that the' movements of the 

 earth's crust over the area of the British Isles during the deposition 

 of the Upper Silurian rocks were on the whole, like the volcanic 

 action of the same geological interval, of a comparatively feeble 

 kind, and that it was not until after the close of the Silurian 

 period that those great terrestrial commotions arose to which our 

 older Palaeozoic rocks mainly owe their plicated, cleaved, and frac- 

 tured character. That the time during which these disturbances 

 continued was long protracted may be inferred from the relations 

 of the Silurian rocks to those which immediately and conformably 

 succeed them. The shales and sandstones at the top of the Silurian 

 series pass upwards without break into the red sandstones and con- 

 glomerates of the Lower Old Red Sandstone. Yet the rapid change 

 10 the nature of the sediment and its included organic remains 

 sufficiently shows that, though the movements may not have been 

 at first of such violence as to produce actual unconformability in 

 any of the tracts which have yet been studied, the conditions of de- 

 position and of the environment of animal life within the area were 

 nevertheless profoundly modified. The prolific Upper Silurian fauna 



