DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



103 



between the two rocks ; but we find that where the erupted material has been poured 

 out at the surface, it has 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 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. # It rises in the conspicuous hill, 'S Airde Beinne 

 (Sarta Beinn), about two miles south-west from Tobermory, and consists of a coarsely 



»/Cr<y (w?v 



aba 

 Fig. 27. — Section of Neck of Basalt, Bendoo, Ballintoy. aa, Chalk ; b, Neck. 



crystalline dolerite, which becomes rather finer in grain towards the outer margin (fig. 

 28). No bedding or structure of any kind beyond jointing is perceptible in it. 

 Examined in thin sections under the microscope, this rock is found to be another typical 

 ophitic dolerite, consisting of lath-shaped felspars, embedded in augite masses, with here 

 and there wedge-shaped portions of interstitial matter and grains of olivine. Dr Hatch 

 observes that the felspars contain spherical inclusions of devitrified glass, filled with black 

 granules and trichites and that, under a high power, the interstitial matter is seen to 

 consist mainly of greenish-brown isotropic matter, in which are inclosed small crystals of 



Fig. 28.— Section of Volcanic Neck at 'S Airde Beinne, near Tobermory, Mull, aa, bedded basalts; bb, bedded basalts 



altered along the side of vent ; cc, dolerite. 



augite, skeleton-forms and microlites of felspar, sometimes in stellate aggregates, as well 

 as club-shaped, cruciform, arrow-headed, and often crested microlites of magnetite. 



Towering prominently above the flat basalt sheets, this neck has an oval form, 

 measuring about half a mile in length by a quarter of a mile in breadth. Its central 

 portion, however, instead of rising into a rugged hill-top, as in all the other instances 

 known to me, sinks into a deep hollow, which is filled with water, and reminds one of a 

 true crater-lake. The middle of the neck is thus concealed from view, and we can only 

 examine the hard prominent ring of dolerite that surrounds the tarn. That the material 



* Quart. Jour. Geol. Soc, xxx. (1874) p. 264. 



