176 PROCEEDINGS OF THE* GEOLOGICAL SOCCETY. 



There seems to have been commonly a contraction and subsi- 

 dence of the materials in the vents, with a consequent dragging 

 down or sagging of the rocks immediately outside, which are thus 

 made to plunge steeply towards the necks. 



When the vents were plugged up by the consolidation of frag- 

 mentary matter or the uprise of lava in them, the final efforts of the 

 volcanoes led to the intrusion of sills and dykes not only into the 

 rocks beneath the volcanic sheets, but also in many instances into 

 at least the older parts of the sheets themselves. These sub- 

 terranean manifestations of volcanic action may be recognized in 

 almost every district. They vary greatly in the degree to which 

 they are developed. Sometimes, as in the Cader Idris, Arenig, and 

 Snowdon regions, they attain an extraordinary importance, alike as 

 regards the number of the sheets and the thickness of some of 

 these. In other cases they are exhibited on so small a scale that 

 they might be overlooked, as in the tract of Carboniferous puy- 

 eruptions in the north of Ayrshire. But they are so generally 

 present as to form a remarkably characteristic feature of our vol- 

 canic districts. 



It is obvious that the time of intrusion of the sills cannot be 

 precisely determined. They were not likely to be injected at an 

 epoch when the volcanic magma could find egress to the surface. 

 That they did not arise before such egress was obtained may be 

 inferred (1st) from their petrographical characters, which are usually 

 those of the later and not of the earlier outflows of the magma ; 

 and (2nd) from the fact that they not only lie among the rocks 

 below the volcanic series, but intersect the lower parts of that 

 series, sometimes even the higher parts. We may therefore, with 

 every probability, regard the sills as among the closing phases of a 

 volcanic period. 



10. The vertical thickness of volcanic material erupted during 

 a volcanic period shows, on the whole, a progressive decrease during 

 Palaeozoic time. In the earlier ages vast piles of lavas and tuffs 5000 

 or 6000 feet in thickness were frequently heaped up, while in later 

 ages the accumulations seldom reached a tenth part of that amount. 



By the intercalation of the volcanic sheets among ordinary sedi- 

 mentary strata, a definite beginning and end to the eruptions are 

 marked. We see exactly where in the stratigraphical series the 

 first showers of ashes fell, and where the last mingled with the 

 ordinary sand and mud of the sea-floor. The same record shows 

 that the volcanic accumulations were finally washed down, that 



