105 THE ANCIENT VOLCANOES OF DERBYSHIRE. 
stone without reaching the bottom. The probability is strong that 
this shaft has been sunk upon a vent, or a large pipe of communica- 
tion with the interior. 
Having said thus much of the vents,a few remarks may be 
next made in regard to the beds of Turr, the Lava-rLiows, and the 
SILLS. 
Or the TUFFs, some appear to be associated with lava-flows, but 
more frequently they are not so accompanied. They are usually 
interbedded with limestone strata, a fact which conclusively demon- 
strates their contemporaneous origin. One of the best examples of a 
tuff without lava occurs at Ashover, where there is an inlier of moun- 
tain limestone similar to those of Kniveton and Crich. This bed is 
known to be over 200 feet thick,and it may be much more. In 
Cressbrook Dale there is a deposit of tuff, separated by nearly 20 
feet of limestone, from a lava-flow about 15 feet thick lying below. 
Other deposits occur in the neighbourhood of 'Tideswell and Tissing- 
ton. That at Grange Mill has already been referred to. 
With regard to the Laya-rLows, none of them appear to be of 
any very great extent, the volcanoes from which they were ejected 
being probably small. The number of vents in such a small area, 
by affording many outlets, probably prevented the formation of one 
great centre of eruption for the whole district. Some of these lava- 
flows are exposed at the surface, while others are deeply buried below 
limestone subsequently deposited. That these concealed flows were 
poured out on the surface is testified by the amygdaloidal and vesic- 
ular nature of the rock, as well as by other indications. One such 
flow passes through the heart of Crich Hill, where it has been pene- 
trated in several places by mine shafts, at a depth of between 400 
and 500 feet. This flow varies greatly in thickness; at one place it 
was found to be quite 120 feet, while at others it was only half that 
thickness, or even much less than that. 
