A. J. Jukes-Browne — Lower Carboniferous in Devon. 355 



of Plymouth. Of this district Mr. Ussher wrote in 1901 ^ : — 

 " Contemporaneous igneous rocks occur in the Upper Devonian in 

 the immediate vicinity of the Wearde St. Erney and Efford 

 sandstones, and these igneous rocks correspond to the lower part of 

 the Devono-Carboniferous volcanic group." Again, he says that 

 both near Saltash and Tavistock " volcanic rocks have been proved 

 to occur in the Upper Devonian and throughout the Lower Culm- 

 measures, but not in the middle Culm-measures." ^ 



Finally, near Beer Alston, Middle Culm sandstones are found to 

 rest directly on Upper Devonian slates, and to explain this he 

 invokes local unconformity due to the volcanic disturbances in the 

 district, remarking that " it is probable that the upheaval of the 

 sea-bed was so irregular in the volcanic districts that Middle Culm- 

 measures, elsewhere apparently conformable to the Lower Culm- 

 measures (as regards position), were partly formed by denudation of 

 the Lower Culm-measures, and so rest on Upper Devonian rocks." 



The existence of volcanic rocks interbedded with the Lower 

 Culm-measures has been known for many years ; they have been 

 described by Mr. Eutley,^ who seems to have thought that Brent 

 Tor was the principal volcanic vent, and that it might have formed 

 a volcanic island in the Carboniferous sea. Similar rocks (tuffs, 

 schalsteins, diabase, and dolerite) occur throughout the range of the 

 Lower Culm, from Launceston on the west to Oakhampton and 

 Belston on the east, and again still further east in the Ashton and 

 Trusham district, where there is a marked development of tuffs and 

 dolerites in the group that contains the Chert - beds. These 

 intercalations are shown on Sheet 339 of the Geological Survey Map 

 (new series). 



From the passages above quoted, and from others which might be 

 given, it is clear that Mr. Ussher connects the volcanic disturbances 

 and eruptions with " a general elevation of the sea-bed either after 

 or during the deposition of the Lower Culm rocks," ^ and the 

 production of a local discordance near St. Mellion. In this view he 

 is very likely to be correct, for it is well known that upheaval 

 is a frequent accompaniment of volcanic activity ; but it seems to me 

 that the supposition of such a movement will explain much more 

 than a mere local unconformity. It will surely explain the small 

 thickness and the character of the deposits which constitute the 

 Upper Devonian and the Lower Culm-measures in South Devon. 



With regard to the Upper Devonian the notable absence of grits 

 or sandstones seems to indicate that the area of deposition was at 

 some distance from any large area of land, while the small, sporadic, 

 and lenticular developments of limestone show that the conditions 

 were not favourable for the growth of calcareous organisms. When 



1 Trans. Inst. Mining Eng., vol. sx, p. 9 of reprint. 



^ The Wearde sandstones and associated volcanic rocks are now included in the 

 Upper Devonian. 



^ Quart. Journ. Geol. Soc, vol. xxxvi, p. 284. 



^ Proc. Soni. Arch, and Nat. Hist. Soc, vol. xlvi, p. 51 of reprint. 



