THE ANCIENT VOLCANOES OF DERBYSHIRE, 106 
Other instances occur of still more extensive lava-flows, but 
descriptive details cannot be given here. Probably those of the Mat- 
lock district and Miller’s Dale covered the greatest area. Each of 
these was at least five miles in length and more than a mile in 
breadth. At times the flow appears to have passed immediately over 
the limestone; in other cases it was preceded by a fall of volcanic 
ash, as in the Miller’s Dale district. Not infrequently a deposit of 
ash succeeded the lava, as for example near Matlock Bath. It will, 
of course, be understood that in each case the ash has been converted 
into a compact mass of tuff. 
The lavas themselves constitute a kind of Dolerite—a rock very 
nearly allied to Basalt—frequently black or dark-green, but greatly 
changed in colour where exposed to the weather. The top and 
bottom of a bed are usually full of holes formed by the gases con- 
tained in the lava stream. The vesicles are often filled with calcite 
by subsequent infiltration, thus forming amygdaloids, which vary 
much in shape and size. 
The more central parts of the beds are harder and free from 
vesicles. 
Of S1xuus or InTRUSIVE SHEETS, there appear to be several exam- 
ples amongst the “‘ Toadstones.” They are not always easy to distin- 
guish from lava-flows, because, while they differ in some respects, 
they possess several features in common witha true surface-flow. 
The chief points of distinction are (1) their frequent unconformity 
to the rocks among which they lie; (2) their connection with some 
channel of supply communicating with a deep-seated source; (3) 
their habit of sending small branches into the rocks above and below; 
(4) their lack of cellular cavities or amygdaloids; (5) their greater 
closeness of texture at top and bottom of the sheet; and, (6) their 
metamorphic action on the rocks above and below them. 
