164 PROCEEDi:XGS OF THE GEOLOGICAL SOCIETY. 



common characters as one great series. Among these links of 

 connexion are the prevalent trend of the dykes in a north-westerly 

 or westerly course, their increase in numbers as they are traced 

 towards the volcanic plateaux of Antrim and the Inner Hebrides, 

 the fact that they traverse the important faults of the country, 

 together with every geological formation, from the Chalk inclusive, 

 downwards, and even the older parts of the basalt-plateaux. They 

 run in long straight courses, sometimes for 60, 70, or even in one 

 case more than 100 miles, though in the region of the plateaux they 

 are much shorter and less regular in breadth and straightness. 



There is good evidence that the dykes are not all of one age. 

 They cross each other. Some of them are truncated by the acid 

 bosses of the Tertiary series, while others of later date run through 

 these bosses. But there is proof that even those which traverse the 

 older parts of the plateau-basalts, and are therefore of later date 

 than these, must be far older than the latest protrusions of the acid 

 rocks.^ In this manner it can be shown that the abundant north- 

 west and south-east dykes of Antrim and the Inner Hebrides form 

 part of the Tertiary volcanic phenomena. 



This system of parallel dykes raises some interesting problems in 

 the physics of the earth's crust. There must have been first formed 

 a series of parallel fissures which, as they run in long straight lines 

 through the most diverse kind of rocks and even without deviation 

 across some of the most important faults in the country, must 

 have been produced by enormous tension of the crust, connected 

 doubtless with the upward pressure of a vast internal reservoir or 

 series of reservoirs of molten rock. Into these fissures, which arose 

 at successive epochs during the Tertiary volcanic period, the lava 

 from beneath ascended, fiUing them from side to side, and cooling 

 there as dykes. "When we remember the vast area over which this 

 process took place we must admit that, interesting as are the other 

 volcanic phenomena of this geological period, the most striking 

 feature is the existence of these great subterranean lakes of lava 

 and the uprise of the material in so many thousands of fissures. 



Whether any of these dykes succeeded in effecting a communica- 

 tion with the surface over the centre and south of Scotland or the 

 north of England can probably never be ascertained, for any super- 

 ficial outflow of material which may once have existed has appa- 

 rently been entirely removed by denudation. But in the north of 

 Ireland and in the long depression between the Outer Hebrides and 

 1 0]^. cit. p. 69. 



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