I 66 PEOCEEDINGS OE THE GEOLOGICAL SOCIEIT. 



volcanic period. Others filled with agglomerate may also be ob- 

 served. Of these, the largest yet noticed occur outside of the basalt- 

 plateaux. One of them, in Strath, Skye, has a diameter of more 

 than two miles. 



In none of the Palaeozoic volcanic groups are the phenomena of 

 the intrusive sheets or sills more conspicuously and instructively 

 developed than in the Tertiary plateaux. These injected masses of 

 rock may be observed throughout the whole region from the far 

 north of Skye to the coast of Antrim, everywhere presenting the 

 same essential characters, though varying much in number and 

 thickness. Here and there they attain a prodigious development as 

 regards the number of separate sills. This feature is more particu- 

 larly to be noticed among the Jurassic strata underlying the base of 

 the basalt-plateaux in the Inner Hebrides.^ Elsewhere, though the 

 sills are comparativelj' few in number, they attain a great thickness 

 individually, as may be characteiistically noticed in the great cliff 

 of Fair Head in Anfrim. These thicker masses present most 

 interesting examples of segregation-veins and patches, wherein 

 some of the component minerals have come together in coarse 

 aggregates, the augite and magnetite being especially conspicuous. 

 They likewise exhibit instances of the so-called ' contemporaneous 

 veins,' where, into rents of the already consolidated rock, some of 

 the still fluid portion has been injected. These structures are specially 

 deserving of attention by students of the more ancient gneisses, 

 wherein they may be paralleled with remarkably similar forms. 



The material of these sills is usually dolerite, often olivine-bearing 

 and with an ophitic structure. It is precisely the kind of rock we 

 should expect to meet with if Ihe basalt of the plateaux was injected 

 among the rocks at some considerable depth from the surface, and 

 was allowed to cool and consolidate there. The constant association 

 of such sills with the base of the plateaux seems to link them with 

 the volcanic operations that gave rise to these great outpourings of 

 lava. Employing the same reasoning as that which has been made 

 use of in regard to the records of the Palaeozoic volcanic periods, we 

 may infer that the Tertiary basic sills belong to a late part of the 

 plateau-eruptions, when the overlying sheets of basalt had attained 

 such a great aggregate thickness that it was easier for the volcanic 

 magma to force its way between the planes of the Jurassic strata 

 and the lower beds of basalt than to make a passage upward to the 

 surface. 



Besides the sills, the volcanic rocks of the Tertiary period of 



^ See, as illustrations of this structure in Trotternish, Skye, the figures in 

 plate xvii. of the atlas accompanying Macculloch's ' Western Islands.' 



