CHAPTER XXVII 
GEOLOGICAL STRUCTURE OF THE CARBONIFEROUS PUTS OF SCOTLAND 
I. Vents : Relation of the Necks to the Rocks through which they rise — Evidence of the 
probable Suboerial Character ot some of tlie Cones or Piiys of Tufl' — Entombment of 
the \ olcanic Cones and their Relation to the Superficial Ejections. 2. Bedded Tuffs 
and Lavas — Eflects of Suhsecpient DislocAtion.^. 3. Sills, Bosses and Dykes. 
Ihe piiy-type of volcanic bill diller.s widely in one respect from those 
which we have hitherto been considering. In the earlier epochs of volcanism 
within the British area, it is the masses ot material discharged from the 
vent, rather than the vents themselves which arrest attention. Indeed, so 
copiously have these masses been erupted that the vents are often buried, or 
their positions have been rendered doubtful, by the uprise in and around 
them of sills and bosses ot molten rock. But among the Carboniferous puys 
the vent is often the only record that remains of the volcanic activity. In 
some cases we know that it never ejected any igneous material to the siir- 
iace. In others, though it may be tilled with volcanic agglomerate or tiitf, 
there is no record of any shower of such detritus having been discharged 
from it. In yet a third class of examples, we see that lava rose in the 
vent, but no evidence remains as to whether or not it ever flowed out above 
ground. Otlier cases occur where beds of lava or of tuff, or of both together, 
have been intercalated in a group of strata, but with no trace now visible of 
the vent from which they came. The most complete chronicle, preserving 
at once a record of the outflow of lava, of the showering forth of ashes and 
bombs, and of the necks that mark the r^eiits of eruption, is only to he found 
in some of the districts. 
I shall therefore, in the present instance, reverse the order of arrange- 
ment followed in the previous chapters, and treat first of the vents, then of 
the materials emitted from them, and lastly of the sills and dykes. 
i. VENTS 
A large number of vents rise through the Carboniferous rocks of Scotland. 
Some ot these are not associated with any interhedded volcanic material, 
so that their geological age cannot be more precisely defined than by saying 
