242 



H. Deicey $ Dr. J. 8. Flett— 



albitized), and basic rocks rich in albite, called skomerite and maiioesite. 

 In Ireland lleynolds and Gardiner have found typical pillow-lavas, 

 cherts, and diabases of Arenig age. 



In Upper Silurian times no spilitic eruptions have yet been recorded 

 in Britain. 



Devonian. The great development of spilite and schalstein in 

 Cornwall and Devon perhaps began in Middle Devonian times, and 

 attained its maximum in Upper Devonian or Lower Carboniferous 

 times. They are accompanied by albite-diabase, minverite, quartz - 

 diabase, picrite, and quartz-keratophyre. 



The subjoined table shows the facies developed from the magma in 

 each epoch at different localities : — 



Harker (35), Prior (36), and Becke (37) have of recent years 

 established the existence of two great suites of eruptive rocks, the 

 Atlantic and the Pacific, and their conclusions have been accepted by 

 Suess (38). In the British Isles we have many examples of these in 

 past time. The Tertiary eruptions of Scotland are of Atlantic facies ; 

 the Carboniferous of the Scottish Lowlands are Atlantic also, but the 

 Old Red are Pacific, while the Ordovician are of the spilitic type. 



In Devon and Cornwall the Ordovician eruptions were spilitic, the 

 Devono-Carboniferous were spilitic, but the Permian were Atlantic, 

 and characterized by trachytes rich in potash. From this Ave see that 

 the same region may in successive geological epochs be the focus of 

 eruptions of entirely distinct suites of volcanic rocks. 



The Pacific volcanic rocks are generated by epochs of active folding, 

 the Atlantic by vertical movements attended by faulting. The 

 spilitic group is characteristic of off-shore subsidences. They are the 

 characteristic volcanic rocks of districts that have been undergoing 

 long-continued subsidence (39). Hence they are not found on the 

 land at the present day, and, in the past, their commonest associates 

 are fine black shales, limestones, and radiolarian cherts. 



It is interesting to recall that in Lower Ordovician times, while the 

 spilites and keratophyres of Southern Scotland and the South of 



