32 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
As the records of the earliest eruptions during the Carboniferous Lime- 
stone period in the district of the Isle of Man are concealed, so also those 
of the last of the series lie under the sea. Where the highest visible tuffs 
overlie the Poyll Vanish limestones they show no change in the nature of 
the materials ejected, or in the energy of eruption. They lie so abruptly 
on the dark calcareous deposits as to show that a considerable pause in 
volcanic activity was followed by a violent explosion. The same abundant 
grey-green pumice, the same kind of loose crystals of felspar, the same type 
of lava-blocks and bombs as had characterized the foregoing eruptions 
remained as marked at the end. But the further volcanic records caunot 
be perused, and we are left to speculate whether the coast-sections reveal 
almost the whole chronicle, or if they merely lay before us the early 
chapters of a great volcanic history of which the main records lie buried 
under the waves of the Irish Sea. 
4. EAST SOMERSET 
Various limited outcrops of igneous rocks have long been known to 
occur in the eastern part of Somerset. The largest of these lies in the 
midst of the Old Bed Sandstone, on the crest of the axis of the Mendip 
Hills, between Downhead and Beacon Hill. Smaller patches occur in 
the Carboniferous Limestone near Wrington Warren, on the north side of 
Middle Hope, on Worle Hill and at Uphill. These rocks have been mapped 
as intrusive, though some of them have been described as conglomeratic 
or as volcanic breccias. While some of the masses are probably intrusive, 
others appear to be truly contemporaneous with the deposition of the 
Carboniferous Limestone. The highly vesicular basalt of Middle Hope 
looks much more like a superficial lava than an intrusion. Mr. Aveliue 
gave a section showing three alternations of limestone and “ igneous rock ” at 
Middle Hope. A recent examination of that coast-line by Mr. A. Strahan 
shows that there are undoubted tuffs interstratified with the calcareous 
strata. There is thus proof that one or more small volcanic vents were 
in eruption on the floor of the Carboniferous Limestone sea in the neigh- 
bourhood of Weston-super-Mare . 1 
5. DEVONSHIRE 
The change from the typical Old Bed Sandstone of South Wales to the 
Devonian system of Devonshire, to which I have already referred, is hardly 
more striking than the contrast between the Carboniferous formations of 
these two areas . 2 The well-marked threefold subdivisions of Carboniferous 
Limestone, Millstone Grit and Coal-measures, so persistent throughout 
Britain, and nowhere more typically developed than in South Wales, are 
1 See Geological Survey Memoir “ On East Somerset,” by H. B. Woodward, 1876, and authorities 
there cited. Mr. Aveline’s section above referred to will be found on p. 22. 
2 In the centre of England numerous outlying areas of igneous rocks are found in the 
Carboniferous Limestone, Millstone Grit and Coal-measures. These will be considered by 
themselves in Chap, xxxii. 
