THE ANCIENT VOLCANOES OF DERBYSHIRE, 102 
conspicuous, as it does in some other parts of the volcanic district. 
At Shothouse Spring the tuff is well exposed, and probably here, as 
well as at other places in the neighbourhood, the change of the tuff 
into clay is the cause of the numerous springs which rise to the 
surface. The weathering of the igneous material may also have 
much to do with the production of the fine pasturage which has Jed 
to the repute possessed by this district in the manufacture of cheese. 
Two small dykes of dolerite, so fine-grained as to be more nearly 
like basalt, pass through the margin of the larger vent, and these 
may probably be continued into the heart of the dome. Fragments 
derived from these dykes may be found in the walls bounding the 
roadside, and in structure the material can hardly be distinguished 
from the close-grained basalts of the Giant’s Causeway. 
No opinion seems to have been advanced as to the probable 
geographical conditions surrounding these volcanoes at the time of 
their actual eruption, and one feels somewhat diffident at expressing 
an opinion when others far more able to propound a theory appear 
to hesitate to do so,. I venture to think, however, rightly or wrongly, 
that the evidences offered by the Grange Mill Vents seem to indicate 
that they occupied a portion of a submarine ridge of limestone,which 
in this particular district reached just above the level of the sea. 
Cones of considerably greater altitude than the hills which now 
represent them were formed in much the same way as volcanic cones 
are developed in the present day. Very little liquid lava character- 
ized the eruptions, but enormous quantities of fragmentary material 
were thrown out over the adjacent sea, where they accumulated on 
the bottom, at times so rapidly as to entirely interfere with the 
deposition of calcareous material, at other times becoming mixed 
with it when discharged in comparatively small quantities. The 
occasional appearance of true lava is proved by the presence of the 
dykes referred to; but whether this lava was merely intrusive, or 
that it reached the surface and was discharged as a lava-flow, is stil 
