Four miles to the south-west of Hopton, on the slope of the hill at Kniveton Wood, another remarkable mass of agglomerate forms a rounded ridge between the two forks of a small stream.[36] Its granular matrix, like that of the other necks, consists of lapilli of minutely vesicular basic glassy lava or pumice, and encloses large and small rounded blocks of finely cellular basalt and pieces of limestone. The rock is unstratified, and in all respects resembles that of ordinary Carboniferous necks in Scotland. Its relations to the Yoredale rocks are laid bare in the channels of the streamlets. There the shales and thin limestones may be seen much broken and plicated, their curved and fractured ends striking directly at the agglomerate. They may be traced to within a yard of the agglomerate. On the Geological Survey map the igneous rock is represented as bounded by two parallel faults. But I hardly think that this explanation suffices to account for the relations of the rocks and their remarkable boundary-line, which seems to me to be undoubtedly the wall of a volcanic vent. To the east of the streams, another mass of agglomerate may mark another neck, while to the north, a third detached area of the same kind of rock, rising among the limestones, may be regarded as likewise a distinct mass. At this locality, therefore, there are two, possibly three, vents. One of these, from the way in which it cuts across the Yoredale shales and limestones, is to be assigned to a time later than the older part of the Yoredale series, and thus, like the Hopton mass, it indicates that in the south of the volcanic area eruptions did not cease with the close of the deposition of the thick limestones, but were prolonged even into the time of the Yoredale rocks.
[36] Outcrop No. 56, p. 638 of Mr. Bemrose's paper.
A further proof of the late age of these southern patches of volcanic material is shown by two bands of vesicular toadstone in the Yoredale series, a little south from the village of Kniveton. These rocks are traced on the Survey Map, and are shown in a diagram in the Memoir, where their position is sought to be explained by a system of parallel faulting.[37] I was able to trace the actual contact of the western band with the strata underneath it, and satisfied myself that there is no fault at the junction. The igneous material is regularly bedded with the Yoredale shales and limestones. Either, therefore, these bands are intercalated lava-streams or intrusive sills. If mere vesicular structure were enough to distinguish true outflowing lavas, then there could be no doubt about these Kniveton rocks. But this structure is found in so many Carboniferous sills, particularly in those thin sheets which have been injected into coals and black shales, that its presence is far from decisive. The vesicles in the Kniveton rocks are small and pea-like, tolerably uniform in size and shape, and crowded together. They are thus not at all like the irregular cavities in the ordinary cellular and scoriaceous lavas of the toadstone series.
Whether or not the question of their true relations be ever satisfactorily settled, these Kniveton bands are certainly younger than the lower portion of the Yoredale group. Their evidence thus agrees with that of the southern agglomerates in showing that the volcanic activity of this region was continued even after the thick calcareous masses of the Carboniferous Limestone series had ceased to be deposited.
Besides the six necks to which I have referred, a rock in Ember Lane, above Bonsall, probably belongs to another vent.[38] It is particularly interesting from the great preponderance of limestone fragments in it. The volcanic explosions at this locality broke up the already solidified limestones on the floor of the Carboniferous Limestone sea, and strewed them around, mingled with volcanic blocks and dust of the prevailing type.
[38] This is outcrop No. 39 of Mr. Bemrose's paper, p. 632.
When the district has been more carefully searched, other centres of eruption will no doubt be discovered. It may then be possible to depict the distribution of the active vents, and to connect with them the outflow of the bedded lavas. So far as I have been able to ascertain, there are no necks of dolerite or basalt, though, as I have shown, dykes or veins of molten rock are occasionally to be found in the agglomerates of the necks.
4. THE LAVAS AND TUFFS.—I have referred to the opinion of De la Beche that the toadstones of Derbyshire were poured out as lava-streams without any accompanying fragmentary discharges, and to the correction of this opinion by the subsequent observations of Jukes and of the Geological Survey. But though the existence of interbedded tuffs has long been known, it was not until Mr. Bemrose's more careful scrutiny that the relative importance of the tuffs among the lavas was first indicated. He has shown that a number of the bands mapped as "toadstone" are tuffs, and he has discovered other bands of tuff which have not yet been placed on any published map.
In examining the outcrops of the various toadstones of Derbyshire we learn that some of them are lavas without tuffs, probably including a number of bands, which are really sills; that others are formed of both lavas and tuffs, and that a third type shows only bedded tuff. Each of these developments will deserve separate description. But before entering into details, we may take note of the varying thicknesses of the different toadstones which have been determined by observation at the surface or by measurement underneath in mining operations. In some cases a distinct band of toadstone, separated by many feet or yards of limestone from the next band, and therefore serving to mark a separate volcanic discharge, may not exceed a yard or two in total thickness, and from that minimum may swell out to 100 feet. The majority of the bands probably range between 50 and 100 feet in thickness. In one exceptional case at Snitterton, a mass of "blackstone" is said to have been proved to be 240 feet thick, but this rock may not improbably have been a sill.[39] The true contemporaneous intercalations seem to be generally less than 100 feet in thickness.