A consideration of the singularly slag-like structure of the injected masses in the tuffs and agglomerates leads to the conclusion that though what we now see of these rocks did not actually flow out at the sea-bottom in streams of lava, it was intruded so close to the surface that the imprisoned vapours had opportunity to expand, as in superficial outflows.[59] This inference is in accord with that derived from an examination of the necks, wherein we find evidence of the probable survival of parts of the actual craters and volcanic cones.
[59] As illustrative of the occurrence of the vesicular structure in superficial intrusions, I may again cite the dyke which cuts the ash of the outer crater-wall of the Puy de Pariou in Auvergne. The andesite of this dyke is in places as vesicular as the lava-stream with which it was doubtless connected, but the vesicles have been flattened and drawn out parallel to the walls of the dyke. In this instance it is quite certain that there could never have been any great depth of detrital material above the fissure into which the material of the dyke was injected (see [vol. i. p. 66]).
As the records of the earliest eruptions during the Carboniferous Limestone 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 Vaaish 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 cannot 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 Red 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. Aveline 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 neighbourhood of Weston-super-Mare.[60]
[60] 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].
5. DEVONSHIRE
The change from the typical Old Red 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.[61] 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 replaced in a distance of less than forty miles by the peculiar "Culm-measures" of Devonshire—a series of black shales, grey sandstones and thin limestones and lenticular seams of impure coal (culm), which are not only singularly unlike in original characters to the ordinary Carboniferous formations, but have been made still more unlike by the extensive and severe cleavage to which the Palæozoic rocks of Devon and Cornwall have been subjected. That these Culm-measures are truly Carboniferous is made abundantly clear by their fossil contents, though it has not yet been possible to determine how far they include representatives of the great stratigraphical subdivisions in other parts of the country.
[61] 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.
It is to De la Beche that geology owes the first intimation of the occurrence of interstratified igneous rocks in the Carboniferous series of Devonshire. As far back as the year 1834, in his singularly suggestive treatise, Researches in Theoretical Geology, this eminent geologist expressed his opinion that not only were the "trappean" bands regularly intercalated in the sedimentary series and continuously traceable with the general stratification, but that they occurred at various localities in such a manner as to raise the suspicion that these points may mark some of the centres of eruption. He particularly cited the example of Brent Tor as a remarkable volcanic-looking hill, composed in part of a conglomerate "having every appearance of volcanic cinders."[62]