Xcii PROCEEDIXGS OF THE GEOLOGICAL SOCIETY. [vol. Ixxili, 



The igneous manifestations were closely connected with these 

 vei*tical disjDlacements, and the manner in which the uprise of 

 molten magma seems to be correlated with the sinking of faulted 

 blocks of country suggests that hydrostatic equilibrium was an 

 important factor in the process. In the British area the maximum 

 activity was developed within a sunken tmct lying between the 

 ancient rocks of the Long Island on the one hand and the main- 

 land of Scotland on the other. Such a trough, however, is in no 

 wise compai*able with that, for instance, of the Midland Valley of 

 Scotland in Carboniferous times, for it does not bear any ascertain- 

 able relation to the geological structm*e of the country Avhich it 

 tm verses. It is an 'Atlantic,' as contrasted with a ' Pacitic ' featm-e. 

 Moreover, in respect of the more important events in the igneous 

 record, this area was merely part of a vast region affected b}^ the 

 same activity. 



These events characterized by a regional extension may be 

 summarized as: first, the outpouring of enormous quantities of lava j 

 second, the intrusion of innumerable sills ; and, third, the injection 

 of a great system of dykes. The lavas, which can be assigned to 

 an Eocene age, did not emanate from great central volcanoes, but, 

 as Sir Archibald Greikie has shown, welled up through innumerable 

 lissures. The individual Hows Avere not large, but collectively they 

 covered a very extensiA^e region. Indeed, for any evidence to the 

 contrary, a continuous lava-field may have stretched from Antrim 

 to beyond Franz Jossf Land, a distance of 2,000 miles. The 

 free opening of so many fissures may be taken as proof that 

 there was at this time no crustal stress involving the element of 

 latei-al pressure. The same conclusion may be drawn from the 

 behaviour of the sills which followed, remarkable for their regu- 

 larity and often of wide extent. Vertical movement was doubtless 

 renewed from time to time, for both lavas and sills are intersected 

 by considerable faults. Lava-Hows and sills were fed by dykes, 

 but in addition there is a great profusion of d3^kes of later age. 

 Distributed over a large part of Britain, they maintain a common 

 direction, which seldom deviates far from north-west and south- 

 east, and show a total disregard for the structure of the countiy 

 which they traverse. Clearly, their intrusion was controlled by 

 some larger law. 



Petrographically these extrusions and intrusions are characterized 

 by an overwhelming preponderance of basic t^q^es, with an alkaline 

 tendency which is sometimes rather latent than expressed, but 



