274 
THE TERTIARY VOLCANOES 
BOOK VIII 
platform of Old Eed Sandstone, some distance to the north of the present 
edge of the volcanic escarpment (Fig. 298). 
The greater coarseness of grain of the material filling these pipes, 
compared with that of the sheets in the terraces, is only what the very 
different conditions of cooling and consolidation would lead us to expect. 
There is no essential difference of composition between the two rocks. 
Where the erupted material has been poured out at the surface, it lias 
assumed a finely crystalline texture, while, where it has slowly solidified 
within a volcanic pipe at some depth beneath the surface, and where con- 
sequently its component crystals have had more time for development, the 
resulting structure is much more largely crystalline, with a more or less 
complete development of the ophitic structure. 
In the island of Mull, another instance of the same kind of vent has 
been observed and described by Professor Judd. 1 It rises in the conspicuous 
hill, ’S Airde Beinne (Sarta Beinn), about two miles south-west from Tober- 
mory, and consists of a coarsely crystalline dolerite, which becomes finer in 
grain towards the outer margin (Fig. 299). No bedding, or structure of 
any kind beyond jointing, is perceptible in it. Examined in thin sections 
Fig. 299.— Section of Volcanic Neck at ’S Airde Beinne, near Tobermory, Mull. 
a a, bedded basalts ; b b, bedded basalts altered along the side of vent ; c c, dolerite. 
under the microscope, this rock is found to be another typical ophitic 
dolerite, consisting of lath-shaped felspars embedded in augite, with here and 
there wedge-shaped portions of interstitial matter and grains of olivine. 
Dr. Hatch found the felspars to contain spherical inclusions of devitri- 
fied glass, filled with black granules and trichites, and he observed that, 
1 Quart. Jour. Geol. Soc., xxx. (1874), p. 264. 
