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. [Illustration: Fig. 179.--Plan of necks and bedded tuff at Grange Mill, five miles west of Matlock Bath.] [Illustration: 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.]