104 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



the later Devonian ages, if not in subsequent geological time. That 

 volcanic activity accompanied the deposition of the Carboniferous 

 rocks of Devonshire has long been well-known. • 



YIII. CARBONIFEROUS. 



Within the area of the British Isles, the geological record is 

 comparatively full and continuous from the base of the Upper Old 

 Red Sandstone to the top of the Coal Measures. We learn from it 

 that the local basins of deposit in which the later portion of the 

 Old Red Sandstone was accumulated sank steadily in a wide 

 general subsidence, in consequence of which the clear sea of the 

 Carboniferous Limestone ultimately spread across Western Europe 

 from the west coast of Ireland into Westphalia. The Carboniferous 

 period, as chronicled by its sedimentary deposits, was thus a time 

 of slow submergence and quiet sedimentation, terrestrial and marine 

 conditions alternating along the margins of the sinking land accord- 

 ing as the rate of depression surpassed or fell short of that of the 

 deposition of sediment. Such a state of balance among the geological 

 agencies does not seem likely to be accompanied with any serious 

 display of volcanic energy. And indeed throughout the Car- 

 boniferous rocks of Western Europe there, is for the most part 

 little trace of contemporaneous volcanic eruptions. Yet striking 

 evidence exists that, along the western borders of the continental 

 area, in Prance as well as in some parts of Britain, which had for 

 so many geological ages been the theatre of subterranean activity, 

 the older half of Carboniferous time witnessed an abundant and 

 variable display of volcanic phenomena. 



Erom the very beginning of the Carboniferous period to the epoch 

 when the Coal Measures began to be accumulated, the southern half 

 of Scotland continued to be a theatre of active vulcanism. The 

 vents shifted their positions there in the course of that prolonged 

 interval of geological time and gradually grew less energetic, but 

 there does not appear to have been any protracted section of the in- 

 terval when the subterranean activity became entirely quiescent. It 

 is not, however, in Southern Scotland only that the records of Car- 

 boniferous volcanic action are preserved, though they are there most 

 complete. Traces of other vents of eruption during the early half 

 of the Carboniferous period have been found in the Isle of Man, 

 in Derbyshire, in Devonshire, and in the South-west of Ireland. To 

 some of these districts further reference will be made in the sequel. 



