[31] Geol. Surv. Mem. on North Derbyshire, by Messrs. Green and Strahan (1887), p. 104.
In the meantime, we know that the lowest visible bands of toadstone are underlain by several hundred feet of limestone, thus proving that the earliest known volcanic explosions took place over the floor of the Carboniferous Limestone sea, after at least 700 or 800 feet of calcareous sediment had accumulated there. The latest traces of volcanic activity are found in a part of the Yoredale group of shales and limestones which form the uppermost member of the Carboniferous Limestone of this region. But it is not quite clear whether the vesicular diabase found there is interstratified or intrusive. Certainly no contemporaneous tuffs have yet been found among the Yoredale rocks, nor in any higher subdivision of the Carboniferous system, though coarse agglomerates marking the position of vents do traverse the Yoredale group at Kniveton.
It may be remarked that in the district over which the toadstones can be seen, two areas are recognizable, in each of which the exposures of the igneous rocks are numerous, while between them lies an intervening tract wherein there is hardly any visible outcrop of these rocks. The northern and much the more extensive area stretches from Castleton to Sheldon, while the southern spreads from Winster to Kniveton. This distribution not improbably points to the original position of the vents, and indicates a northern more numerous group of volcanic orifices, and a southern tract where the vents were fewer, or at least spread their discharges over a more limited space.
3. THE VENTS.—It had always appeared to me singular that, in ground so deeply trenched by valleys as the toadstone district of Derbyshire, no trace had been recognized of any bosses or necks from which these volcanic sheets might have been erupted. It is true that in mining operations masses of toadstone had been penetrated to a considerable depth without their bottom being reached, and the suggestion had been made that in such cases a shaft may actually have been sunk on one of the vents through which the toadstone came up.[32] One instance in particular was cited where, at Black Hillock, on Tideswell Moor, close to Peak Forest Village, a mass of toadstone was not cut through, though pierced to a depth of 100 fathoms. In that neighbourhood, however, several of the sheets of eruptive material are probably sills, and the shaft at Black Hillock may have been sunk upon the pipe or vein that supplied one or more of these intrusive sheets.
[32] Geol. Surv. Mem. on North Derbyshire, p. 134.
It was therefore with no little interest that I detected a series of vents at four separate localities, viz. Castleton, Grange Mill, Hopton, and Kniveton Wood. I have no doubt that a more extended search will bring others to light. Those observed by me are all filled with coarse agglomerate, the blocks in which are mostly composed of different lavas, sometimes with the addition of blocks of limestone, while the matrix consists mainly of lapilli of basic devitrified glass.
The most typical examples form a group of two, possibly three, vents which rise into two isolated, smooth, grassy dome-shaped hills at Grange Mill, five miles west from Matlock Bath.[33] In external form and colour, these eminences present a contrast to the scarped slopes of limestone around them. They at once recall the contours of many of the volcanic necks in Central Scotland. On examination it is found that the material composing them is a dull green agglomerate, the matrix of which is a compact substance weathering spheroidally, and full of small lapilli of minutely vesicular diabase. The larger stones consist, for the most part, of various vesicular dolerites or diabases, together with some pieces of limestone and occasionally large blocks of the latter rock, altered into a saccharoid condition. Two dykes of dolerite or basalt traverse the margin of the larger vent.
[33] This is Mr. Bemrose's outcrop, No. 46, op. cit. p. 633.
The steep sides of these agglomerate domes rise from the low ground around them to a height of 100 to 180 feet, their summits being a little more than 900 feet above the sea. The smaller neck is nearly circular, and measures about 1000 feet in diameter. The larger mass is less regular in shape, and is prolonged into such a bulge on the south-east as to suggest that its prolongation in that direction may really mark the position of a third and much smaller vent contiguous to it. The longer diameter of the larger mass is 2300 and the shorter 1300 feet.
