i6 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
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 . 1 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 he 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 
he 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. 
Along the southern margin of the limestone country a group of agglo- 
merate 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 , 2 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 lime- 
stone, 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. 
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 . 8 Its granular matrix, like 
that of the other necks, consists of lapilli of minutely vesicular basic glassy 
1 This is outcrop No. 1 of Mr. Bemrose’s paper, p. 625. 
3 Geol. Surv. Mem. North Derbyshire, p. 24. This is outcrop No. 53 of Mr. Bemrose's paper, 
p, 635. 3 Outcrop No. 56, p. 638 of Mr. Bemrose’s paper. 
