107 THE ANCIENT VOLCANOES OF DERBYSHIRE. 
But these distinctive characters are not always present, and 
much careful examination and considerable field-experience are 
neeessary before a conclusion can be arrived at. 
Probably one of the most typical and best authenticated sills of 
the district is that of Tideswell Dale, which is exposed about half-a- 
mile South of the village of Tideswell. Among other points of inter- 
est to be noted here, is the presence of a bed of clay, probably a 
decomposed Toadstone, or it may be a volcanic mud, covered in 
places by the sill, which has indurated it and even caused columnar 
structure to the depth of several feet. The limestones, too, are con- 
siderably altered by contact, becoming more or less crystalline in 
proximity to the sill. 
In the same locality there is also a lava-flow, which undoubtedly 
(judging from relative position, &c,) preceded the intrusion of the 
sill; and this flow has not changed the character either of the clay 
or the limestone. 
