CHAP. XL 
FISSURE ERUPTIONS 
269 
among the basalt-sheets, but I have never found a case which could he 
confidently cited as an example of lava rising in a fissure and spreading- 
out as a superficial sheet. That this connection may eventually be found 
when a more detailed survey is made of these great sea-walls I fully 
anticipate. 
In recently mapping the basalt-plateau of Strathaird in Skye, Mr. 
Harker has made some interesting observations regarding the probable con- 
nection of the dykes with the plateau basalts. He has noticed that the 
flanks of Slat Blieinn, a portion of the plateau, are abundantly traversed 
by dykes containing numerous enclosed pieces of gabbro, while the basalt 
on the summit of the plateau is full of similar fragments — an occurrence 
not observed elsewhere. It is conceivable that the gabbro-bearing basalt- 
sheets are sills, but Mr. Harker has found no proof that they are so, the 
evidence so far as it has been collected being rather in favour of the view 
that these sheets are superficial lavas, and that they have been supplied 
from the dyke-fissures. 
Various considerations suffice to assure us that actual instances of the out- 
flow of the basalt from its parent fissures should be expected to be excep- 
tional. The absence or scarcity of beds of scoria-, among the basalt-plateaux 
may be taken as an indication that the lava as a rule flowed out without 
the formation of cinder-cones, and therefore that these conspicuous monu- 
ments of the eruptive vents were probably always rare in Britain. If the 
lava was poured out tranquilly from one or two points along a fissure which 
were subsequently buried under floods of similar lava issuing from other 
fissures, the chances that such points of emission should be laid open along 
the front of any escarpment are small. And, even when so exposed, it 
might be difficult to feel sure that the dyke below was really the feeder of 
the basalt above, unless the cliff were accessible and the rocks could be 
scrutinized foot by foot. These elements of uncertainty are happily 
removed where the volcanic energy has drilled well-marked funnels of dis- 
charge and left them filled with the erupted materials, as will be narrated 
in the next chapter. 
