Vol. 63.^ THE TOADSTOKES OF DERBYSHIRE. 263 



them penetrates the agglomerate only to a height of several feet 

 above the level of the road. 



The coarser parts of the largest dyke consist of an olivine-dolerite 

 with granular augite, in which towards the margin the felspars 

 become smaller. The difference in structure between the central 

 portions and the margin is well shown in the slices from one of the 

 smaller dykes. The felspars are as big as those in the central parts 

 of the largest dykes, but higher up they become smaller, and the 

 margin of the dyke in contact with the agglomerate consists of 

 small felspar-laths in a brown and non-isotropic base. The felspar- 

 laths are arranged with their sides parallel to the edge of the 

 dyke. 



A well-marked escarpment of limestone, consisting of beds at 

 least several hundred feet higher in the series than those surrounding 

 the vents, may be seen on both sides of the valley. Immediately 

 below this escarpment is a bed of greenish volcanic tuff, consisting 

 of fine and coarse layers of lapilli in a calcareous cement. I 

 have traced this tuff from near Whitelow Farm on the south- 

 east, by Shothouse Spring on the north, to within a short distance 

 from the road between Grangemill and Aldwark on the south-west, 

 as indicated on the map (PL XXI). It apparently thins out on the 

 south-west and south-east. 



The limestone resting upon the tuff is nodular or concretionary 

 in character, and contains lapilli up to a height of at least 18 feet, 

 thus showing that the volcanic action died out gradually and that 

 the detritus was mingled with the limestone of the sea-floor. 



Although it is at present impossible to determine the horizon of 

 these vents, there is no doubt that the bed of tuff is some distance 

 below the lower lava-flow of the district. The Grangemill Vents 

 are, therefore, the oldest in the Matlock area. 



Ember-Lane Vent. — Immediately to the east of the village of 

 Bonsall, a coarse agglomerate is seen exposed in the steep banks of 

 Ember Lane and in the fields above it on the north-west. The vent 

 is elliptical in area, and extends for a distance of about 1000 feet 

 from north-west to south-east, and for about 400 feet at right angles 

 to that direction. Seen from the opposite side of the valley, it makes 

 a distinct feature on the hill-slope. There is unfortunately no 

 exposure of limestone close to the agglomerate ; but the beds about 

 300 or 400 feet on the south-west dip south-eastward, and those on 

 the north dip nearly due east. In the more immediate neighbourhood 

 we have on the north-west an intrusive dolerite, on the south-east 

 a lava-flow, and on the north a quartz-rock which I have shown in 

 a previous paper to be a silicified limestone. 1 



The stratigraphical evidence in favour of its being a vent is, there- 

 fore, not so clear as in the case of other vents in the county. The 

 nature of the agglomerate, the distinct although slight feature 

 which it presents, and its relations to the surrounding rocks are 



1 Quart. Journ. Geol. Soc. vol. liv (1898) pp. 169-82 & pis. xi-xii. 



