68 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



and partly on the number of separate dykes that are diverted into the line of transverse 

 fissure ; for, where the fissure crosses an area with fewer N.W. dykes, the band becomes 

 thinner or ceases altogether. 



In some rare cases, the dykes have been shifted by more recent faults. I shall have 

 occasion to show that faults of several hundred feet have taken place since the Tertiary 

 basalt-plateaux were formed. There is therefore no reason why here and there a fault 

 with a low hade should not have shifted the outcrop of a dyke. But the fact remains 

 that, as a general rule, the dykes run independently of faults even where they approach 

 closely to them. Mr Clough has observed some interesting cases in south-eastern 

 Argyleshire, where the apparent shifting of a dyke by faults proves to be deceptive, and 

 where the dyke has for short distances merely availed itself of old lines of fracture. 

 One of the most remarkable of these is presented by the large dyke which runs westward 

 from Dunoon. No fewer than three times, in the course of four miles between Lochs 

 Striven and Riddon, Mr Clough has found this dyke to make sharp changes of trend 

 nearly at right angles to its usual direction, where it encounters N. and S. faults (fig. 

 17). It would be natural to conclude that these changes are actual dislocations due to 

 the faults. But this careful observer has been able to trace the dyke in a very attenuated 

 and uncrushed form along some of these cross faults, and thus to prove that the faults 

 are of older date, but that they have modified the line of the long E. and W. fissure up 

 which the material of the dyke ascended. 



§ 17. Data for Estimating the Geological Age of the Dykes. 



I have already assigned reasons for regarding the system of E. and W. or S.E. and 

 N.W. dykes as belonging to the Tertiary volcanic period in the geological history of the 

 British Islands. But I have no evidence that they were restricted to any part of that 

 period. On the contrary, there is every reason to consider the uprise of the earliest 

 and latest dykes to have been separated by a protracted interval. That they do not 

 all belong to one epoch I shall now proceed to prove. 



The intersection of one dyke by another furnishes an obvious criterion of relative age. 

 Macculloch drew attention to this test, and stated that it had enabled him to make out 

 two distinct sets of dykes in Skye and Rum. But he confessed that it failed to afford any 

 information as to the length of the interval of time between them.* It is not always so 

 easy as might be thought to make sure which of two intersecting dykes is the older. 

 We have to look for the finer-grained marginal strip at the edge of a dyke, which, where 

 traceable across another dyke, marks at once their relative age. The cross joints of the 

 two dykes also run in different directions. It is obvious that in the case of two such 

 dykes, no longer interval need have elapsed between their successive production than was 

 needed for the solidification and assumption of a joint-structure by the older one before the 

 younger broke through it. They may both belong to one brief period of volcanic activity. 

 But when we pass to a series of dykes traversing a considerable district of country, and 



* Trans. Geol. Soc, iii. p. 75. 



