CHAP. XL! 
VENTS OF THE BASALT-PLATEAUX 
■97 
streams of lava which, in flowing over the volcanic fields, eventually buried 
and obliterated each of the vents. 
In the destruction of the precipice some of the vents have been so 
much cut away that only a small part of the wall is left, with a portion ot 
the agglomerate adhering to it. The third neck, lor instance, affords the 
section represented in Tig. 313, where the horizontal sheets of basalt (a) have 
still a kind of thick pellicle of the volcanic detritus ( b ) adhering to what 
must have been part of the side of the orifice of eruption. The waves 
have cut out a cave at the base, so that we can, by boat, get behind 
the agglomerate and see the margin of the volcanic funnel in the root 
overhead. 
The fragment of geological history so picturesquely laid bare on the 
Stromo cliffs presents a significant illustration of what seems to have been 
a frequent, if not the normal type of volcanic, vent in the Tertiary basalt- 
plateaux. By the fortunate accident that denudation has not proceeded 
too far, we are able to observe the original tops of at least two of the vents, 
and to see how such volcanic orifices, which were doubtless abundant all 
over these plateaux, came to be entombed under the ever-increasing pile of 
accumulating basalt. 
There is still one feature of interest in these cliff-sections which deserves 
notice here. Every geologist who has studied the composition of the basalt- 
plateaux has remarked the comparatively insignificant part played by tuffs 
in these, volcanic accumulations. Hundreds of feet of successive basalt- 
sheets may often be examined without the discovery of any intercalation of 
fragmental materials, and even where such intercalations do occur they are 
for the most part quite thin and extremely local. I found it impossible to 
scale the precipice for the purpose of ascertaining whether around the 
Stromo vents, and connected with them, there might not be some beds of tuff 
interstratified between the basalts. If such beds exist, they can only be of 
trifling thickness and extent. Here, then, are examples of once active vents, 
the funnels of which are still choked rqo with coarse fragmentary ejections, 
yet from which little or no discharge of ashes and stones took place over the 
surrounding ground. They seem to have been left as crater-like hollows on 
the bare surface of the lava-fields. 
