DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



31 



infrequent as a black vitreous varnish-like coating on the outer walls, and occasionally 

 appears in strings and veins even in the centre. 



It is the assemblage of dykes presenting these features which I propose to describe. 

 Obviously, the age of each particular dyke can only be fixed relatively for itself. But 

 when this remarkable community of characters is considered, and when the Tertiary age 

 of at least a very large number of the dykes can be demonstrated, the inference is reason- 

 able that the whole assemblage constitutes one great system, extravasated during a time 

 of great volcanic disturbance, which could not have been earlier than the beginning 

 of the Tertiary period. And this inference may be maintained even when we frankly 

 admit that every dyke within the region is by no means claimed as belonging to the 

 Tertiary series. 



'v-«< c ^ w . 



Fig. 1, — Dyke on the south-east coast of the Island of Mull. 



In spite of their number and the extraordinary volcanic activity to which they bear 

 witness, the basic dykes form a much less prominent feature in the landscape than might 

 have been anticipated. In the lowlands of the interior, they have for the most part been 

 concealed under a cover of superficial accumulations, though in the water-courses they not 

 infrequently project as hard rocky barriers across the channels, and occasionally form 

 picturesque waterfalls. On the barer uplands, they protrude in lines of broken crag 

 and scattered boulders, which by their decay give rise to a better soil covered by a 

 greener vegetation than that of the surrounding brown moorland. Among the Highland 

 hills, they are often traceable from a distance as long black ribs that project from the naked 

 faces of crag and corry. Along the sea coast, their peculiarities of scenery are effectively 

 displayed. Where they consist of a close-grained rock, they often rise from the beach as 



