Fig. 178.—View of two volcanic necks in the Carboniferous Limestone series, at Grange Mill, five miles west of Matlock Bath, from the north.
On the south and west sides, the surrounding limestone can be traced up to within a few feet of the edge of the agglomerate, and its strata are there found to be much jumbled and broken, while their texture is rather more crystalline than usual, though not saccharoid. The two necks are separated by a narrow valley in which no rock is visible. Their opposite declivities meet at the bottom of this hollow, and are so definitely marked off that, even in the absence of proof that they are disjoined by intervening limestone, there can be little hesitation in regarding each hill as marking a distinct vent. A wider valley extends along the eastern base of the necks, and slopes upward on its east side until it is crowned by a long escarpment of limestone, which reaches a height of 1000 feet above the sea, or about 100 feet above the valley from which it rises. Unfortunately, the bottom and slopes of this depression are thickly covered with soil, but at one or two places debris of fine tuff may be observed, and at the northern and southern ends of the hollow well-bedded green and reddish tuff appears, dipping gently below the limestone escarpment. This band of volcanic detritus evidently underlies the limestone, and forms most of the gentle slope on the east side of the valley. It may be from 70 to 100 feet thick. That it was discharged from one or both of the necks seems tolerably clear. Its material resembles that forming the matrix of the agglomerate. The general arrangement of the rocks at this interesting locality is represented in Fig. 179, which is reduced from my survey on the scale of six inches to a mile. A section across the smaller vent would show the structure represented in [Fig. 180].
Fig. 179.—Plan of necks and bedded tuff at Grange Mill, five miles west of Matlock Bath.
Fig. 180.—Section across the smaller volcanic neck and the stratified tuff in Carboniferous Limestone, Grange Mill.
1. Limestone; 2. Stratified tuff intercalated among the limestones; 3. Agglomerate.
This group of vents lies in the southern of the two tracts of the volcanic district. In the northern tract a mass of agglomerate pierces the base of the limestone escarpment about a quarter of a mile west from the entrance to the Peak Cavern at Castleton.[34] It is rudely semicircular in area, stretching down the slope until its northern extension is lost under the lower ground. The agglomerate is not well exposed, but it can be seen to be a green, granular crumbling rock, made up in great part of minutely vesicular lapilli, enclosing blocks of various diabases two feet long or more. From the abrupt way in which this agglomerate rises through the limestone, there can be little doubt that it marks the position of one of the volcanic vents of the time. As it stands on the extreme northern verge of the limestone area, the ground further north being covered with the Yoredale rocks and Millstone Grit, it is the most northerly of the whole volcanic district.
[34] This is outcrop No. 1 of Mr. Bemrose's paper, p. 625.
Along the southern margin of the limestone country a group of agglomerate masses probably marks another chain of vents. These are specially interesting, inasmuch as they abut on the Yoredale series, and may thus be looked upon as among the latest of the volcanic chimneys. One of them is seen at Hopton,[35] where along the side of the road a good section is exposed of coarse tumultuous agglomerate, having a dull green matrix, composed of green, brown, and black, minutely cellular, basic, devitrified, glassy lapilli, showing under the microscope abundant microlites and crystals or calcareous pseudomorphs of olivine, augite, and felspar, and much magnetite dust. Through this matrix are distributed blocks of slaggy basalt and dolerite. An interesting feature of this mass is the occurrence in it of some veins, two or three inches broad, of a compact black porphyritic basalt. I did not trace the relations of this agglomerate to the stratified rocks around it. But its internal structure and composition mark it out as a true neck. It extends, according to the Geological Survey map, for about half a mile along the edge of the limestone, and is represented as being separated by two faults from the Yoredale series immediately to the south. So long as the belief is entertained that the toadstones are contemporaneous outflows of lava lying on certain definite horizons, far below the summit of the limestones, the position of the Hopton agglomerate is only explicable on the assumption of some dislocation by which the Yoredale shales have been brought down against it. But when we realize that the rock is an unstratified agglomerate, probably marking the place of a volcanic vent, and therefore rising transgressively through the surrounding strata, the necessity for a fault is removed, or if a fault is inserted its existence should be justified on other evidence than the relations of the igneous rock to the surrounding strata.
[35] Geol. Surv. Mem. North Derbyshire, p. 24. This is outcrop No. 53 of Mr. Bemrose's paper, p. 635.