the carboniferous volcanoes 
BOOK VI 
rom the great preponderance of limestone fragments in it. The volcanic 
explosions at this locality broke up the already solidified limestones on the 
fioor of the Carboniferous Limestone sea, and strewed them around, mingled 
with volcanic blocks and dust of the prevailing type. 
When the district has been more carefully searched, other centres of 
eiup ion will no doubt be discovered. It may then be possible to depict 
t distribution of the active vents, and to connect with them the outflow of 
ie bedded lavas. So far as I have been able to ascertain, there are no 
necks of dolerite or basalt, though, as I have shown, dykes or veins of molten 
rock are occasionally to be found in the agglomerates of the necks. 
. THE LAVAS AND toffs. — I have referred to the opinion of De la 
eche that the toadstones of Derbyshire were poured out as lava-streams 
without any accompanying fragmentary discharges, and to the correction of 
tins opinion by the subsequent observations of Jukes and of the Geological 
Survey. But though the existence of interbedded tuffs has long been known, 
itwas not until Mr Bemrose’s more careful scrutiny that the relative im- 
portance of the tuffs among the lavas was first indicated. He has shown that 
a number of the bands mapped as “ toadstone ” are tuffs, and he has discovered ! 
o rei hands of tuff which have not yet been placed on any published map. 
In examining the outcrops of the various toadstones of Derbyshire 
we learn that some of them are lavas without tuffs, probably including a' 
number of bands, which are really sills ; that others are formed of both 
avas and tuffs, and that a third type shows only bedded tuff. Each of 
these developments will deserve separate description. But before entering 
m o details, we may take note of the varying thicknesses of the different 
toadstones which have been determined by observation at the surface or by 
measurement underneath in mining operations. In some cases a distinct 
band of toadstone, separated by many feet or yards of limestone from the 
next band and therefore serving to mark a separate volcanic discharge may • 
not exceed a yard or two in total thickness, and from that minimum may 
50 ,„d“ 00 « • , TI ' e "T** ° ( 1118 I«M* ™ Se between 
, | tee f m thlckness - In one exceptional case at Snitterton,a mass 
blackstone is said to have been proved to be 240 feet thick but 
this rock may not improbably have been a sill. 1 The true contemporaneous 
intercalations seein to be generally less than 100 feet in thickness. 
r} LaVa ® without Tuffs.— Examples occur of sheets of toadstone which 
consist entirely of contemporaneously ejected diabase, basalt or dolerite. This ; 
roc v is then dull green or brown in colour, more or less earthy in texture, I 
and irregularly amygclaloidal. The vesicles are extremely varied in size, 
orm and distribution, sometimes expanding until the rock becomes a slagoy 
mass. A central more solid portion between a scoriaceous bottom and top i 
“blaok S toS r ”“ 0 Thefo^ 6 ^ tte mini ' ,g bet "' een “ toadstone ” and what is called 
«,lid ,,,d If dUtinotion b. w.ll fc.adeTC m! 
t. ,.,,k th. open -111., l.,„, t„. olhn U» ^ „„ | 
