101 THE ANCIENT VOLCANOES OF DERBYSHIRE. 
been entirely weathered down to the general level of the country. 
An observer, who has seen the voleanic necks of Scotland and North 
Treland, is at once struck by the general resemblance of the Grange 
Mill Vents to these well-authenticated relics of bygone volcanic 
action. ‘The two domes are separated by a narrow valley, in which 
the rocks, probably limestone, are thickly covered by material worn 
down by weather from the slopes. Limestone surrounds the domes 
on every side, forming escarpments on the east and west. The vents 
differ in form and size. The more southerly is the larger, and it is 
somewhat irregular in shape. The smaller vent is almost circular. 
The larger covers an area of something like 72 acres, while the 
smaller occupies about 27 acres. They reach an elevation of between 
100 and 200 feet above the surrounding country, and their summits 
are about 900 feet above sea-level. Both vents are grass-grown to 
the top, and are intersected by hedges dividing them into fields. 
Here and there on the slopes, tracks worn by cattle, and sections 
caused by the falling away of small banks, reveal the nature of the 
material forming the hills. This consists generally of somewhat 
soft greenish rock, containing fragments of much harder dolerite—a 
rock so hard that it can only be broken with difficulty. Other 
fragmentary matter consists of pieces of limestone, some of which 
have been highly crystallized by heat. The matrix is essentially 
voleanic ash. Qn the roadside, where this agglomerate has been 
exposed, it is seen to weather into spheroidal masses, which, when 
broken, are found to contain large numbers of D1asasic Lapriu. At 
some distance from the vents, and passing underneath the upper 
beds of limestone, are to be found deposits of tuff, which are from 60 
to 90 feet in thickness, and these probably cover a large area of the 
ancient sea-bottom in the vicinity of the voleances. As an evidence 
that the eruptions were in progress contemporaneously with the 
deposit of the limestone, the beds lying above the tufts reveal the 
presence of small igneous fragments mixed with the calcareous 
material, This, however, does not occur to such an extent as to be 
