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ANNIVERSARr ADDRESS OF THE PRESIDENT. I03 



piece of any igneous rock can be procured among the lanes and 

 shallow pits where alone, for the most part, the materials are 

 exposed. 



Much therefore remains to be done, both in the stratigraphy and 

 petrography of the Devonian volcanic rocks of this country. So far 

 as at present known, the lavas are mainly or entirely ' greenstones,' 

 varying from a compact to an amygdaloidal or slaggy texture. 

 They occur in sheets, either singly or in groups, and are regularly 

 bedded among the sedimentary strata. Originally they seem to 

 have been dolerites, and among the fresher slides Dr. Hatch has 

 found most of them to possess an ophitic structure, though some 

 show granular and others porphyritic structure. With these rocks 

 are associated abundant diabase-tuffs, frequently mingled with 

 ordinary non-volcanic detrital matter, and shading off into the 

 surrounding grits and slates. There is thus clear evidence of the out- 

 pouring of basic lavas and showers of ashes during the Devonian 

 period in the South-west of England, under conditions analogous 

 to those which characterized the formation of the Devonian system 

 in Nassau and the Harz. 



The exact range of the eruptions in geological time has still to 

 be ascertained. So far as at present determined, volcanic activity 

 was not awakened during the accumulation of the Lower Devonian 

 formations. It was not until the sporadic coral-reefs and shell- 

 banks had grown up, which form the basement limestones of the 

 Middle Devonian group, that the first eruptions took place. As 

 Mr. Champernowne and Mr. Ussher have shown, some of these 

 reefs were overwhelmed with streams of lava or buried under 

 showers of ashes. The volcanic discharges, however, were only 

 local, probably from many scattered vents, and never on any great 

 scale. Some districts remained little or not at all affected by them, 

 so that the growth of limestone went on without interruption, or 

 at least with no serious break. In other areas again the place of 

 the limestone is taken by volcanic materials. 



The chief epoch of this volcanic action marked by the ' Ashpring- 

 ton Volcanic Series ' appears to have occurred about midway in the 

 Middle Devonian period. But in certain districts it may have ex- 

 tended into Upper Devonian time. Some intrusive sills of diabase 

 may mark the later phases of the volcanic history. But the occur- 

 rence of such sills even in the Upper Devonian rocks, and the 

 alteration of the strata in contact with these (spilosite), point to 

 the continuance or renewal of subterranean disturbance even in 



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