70 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



volcanic series, were not all produced at one epoch, but belong to at least two (and 

 possibly to many more) episodes in one long volcanic history. As they rise through 

 every member of that series of rocks (save the pitchstones), some of them must be among 

 the very latest records of the prolonged volcanic activity. But, on the other hand, 

 some may go back to the beginning of the Tertiary volcanic period. 



§ 18. Origin and History of the Dykes. 



Reference has already been made to the doubt expressed by Macculloch whether 

 the dykes of Skye had been filled in from above or from below. That the dykes of 

 the country as a whole were supplied from above, was the view entertained and 

 enforced by Boue. He introduces the subject with the following remarks : — " Scot- 

 land is renowned for the number of its basaltic veins, which gave Hutton his ideas 

 regarding the injection of lava from below; but, as the greatest genius is not 

 infallible, and as volcanic countries present us with examples of such veins arising 

 evidently from accidental fissures that were filled up by currents of lava which moved 

 over them, and as the Scottish instances are of the same kind, we regard it as infinitely 

 probable that all these veins have been formed in the same way, notwithstanding the 

 enormous denudation which this supposition involves ; and that only rarely do cases 

 occur where they have been filled laterally or in some other irregular manner." # I need 

 not say that this view, which, except among Wernerians, had never many supporters, 

 has long ago been abandoned and forgotten. There is no longer any question that the 

 molten material came from below. 



1. In discussing the history of the dykes, we are first confronted with the problem of 

 the formation of the fissures up which the molten material rose. From what has been 

 said above regarding the usual want of relation between dykes and the nature and 

 arrangement of the rocks which they traverse, it is, I think, manifest that the fissures 

 could not have been caused by any superficial action, such as that which produces cracks 

 of the ground during earthquake-shocks. The fact that they traverse rocks of the most 

 extreme diversities of elasticity, structure, and resistance, and yet maintain the same 

 persistent trend through them all, shows that they originated far below the limits to 

 which the known rocks of the surface descend. We have seen that in the case of the 

 Cleveland dyke, the fissure can be proved to be at least some three miles deep. But the 

 seat of origin of the rents no doubt lay much deeper down within the earth's crust. 



It is also evident that the cause which gave rise to these abundant fissures must have 

 been quite distinct from the movements that produced the prevalent strike and the main 

 faults of this country. From early geological time, as is well known, the movements of 

 the earth's crust, beneath the area of Britain, have been directed in such a manner as to 

 give the different stratified formations a general north-east and south-west strike, and 

 to dislocate them by great faults with the same average trend. But the fissures of the 

 Tertiary dykes run obliquely and even at a right angle across this prevalent older series 



* Essai Geologique sur I'Ecosse, p. 272. 



