14 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
them is a dull green agglomerate, the matrix of which is a compact substance 
weathering spheroidally, and full of small lapilli of minutely vesicular 
diabase. The larger stones consist, for the most part, of various vesicular 
dolerites or diabases, together with some pieces of limestone and occasionally 
large blocks of the latter rock, altered into a saccharoid condition. Two 
dykes of dolerite or basalt traverse the margin of the larger vent. 
The steep sides of these agglomerate domes rise from the low ground 
around them to a height of 100 to 180 feet, their summits being a little 
more than 900 feet above the sea. The smaller neck is nearly circular, 
Fig. 178. — View of two volcanic necks in the Carboniferous Limestone series, at Grange Mill, five miles 
west of Matlock Bath, from the north. 
and measures about 1000 feet in diameter. The larger mass is less regular 
in shape, and is prolonged into such a bulge on the south-east as to suggest 
that its prolongation in that direction may really mark the position of a 
third and much smaller vent contiguous to it. The longer diameter of the 
larger mass is 2300 and the shorter 1300 feet. 
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 lime- 
stone, there can be little hesitation in regarding each hill as marking a 
distinct vent. 
