12 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
some of the Carboniferous basic tuffs of Fife, already referred to (vol. i. p. 422), 
and like these they include abundant blocks of dolerite and basalt. 
2. GEOLOGICAL STRUCTURE OF THE TOADSTONE DISTRICT. As the 
volcanic rocks of Derbyshire lie among the Carboniferous Limestones of a 
road anticlinal dome, they are only exposed where these limestones have 
been sufficiently denuded, and as the base of the limestones is nowhere laid 
lare, the lowest parts of the volcanic series may be concealed. Over the 
tract where the toadstanes can be examined they appear as bands 
regularly intercalated with the limestones, but varying in thickness in the 
course of their outcrops. As they are prone to decay, they usually form 
smooth grassy slopes between the limestone scarps, though isolated blocks 
of the dull brown igneous rocks may often be seen protruding from the 
surface. Now and then a harder bed of toaclstone caps a hill, and thus 
orms a prominent feature in the landscape, but as a rule these igneous 
an s . P a ' 110 distinguishing part in the scenery, and are indeed less 
conspicuous than the white escarpments of limestone which overlie them. 
It was the opinion of the older geologists that three distinct platforms 
of toadstone extend without break throughout the district, and subdivide the 
limestones into four portions. But this opinion does not seem to have been 
based on good evidence either of sequence or of continuity. Various facts 
were brought forward by the officers of the Geological Survey to show that 
the supposed persistence of the three platforms of toadstone did not really 
exist, but that these sheets of igneous material are found at different spots 
on very different horizons, and are of limited horizontal, range. 1 So far as 
my own limited observations go, they entirely corroborate this view. There 
can be little doubt, I think, that the identity of certain outcrops of toad- 
stone has been assumed, and the assumption has been carried throughout 
tie district. The truth is that the number of successive platforms on 
which igneous materials appear will never be satisfactorily determined until 
the stratigraphy of the Derbyshire Carboniferous Limestone is worked out 
in detail. When the successive members of this great calcareous formation 
have been identified by lithological and palaeontological characters over the 
district, it will be easy to allocate each outcrop of toadstone to its true 
geological horizon. When this labour has been completed, it will probably 
be found that instead of three, there have been many discharges of volcanic 
material during the deposition of the limestone series; that these have 
proceeded from numerous small vents, and that they are all of com- 
paratively restricted horizontal extent. Such a detailed examination will 
also determine how far the toadstones include veritable sills, and on what 
horizons these intrusive sheets have been injected. 
In the meantime, we know that the lowest visible bands of toadstone 
are underlain by several hundred feet of limestone, thus proving that the 
earliest known volcanic explosions took place over the floor of the 
Carboniferous Limestone sea, after at least 700 or 800 feet of calcareous 
sediment had accumulated there. The latest traces of volcanic activity are 
1 Geol. Swrv. Mem. on North Derbyshire, by Messrs. Green and Strahan (1887), p. 104. 
