232 Dr. Berger on the Dykes 



It Is very evident from these observations that some of the dykes 

 at least were formed at a later period than the metallic veins ; an 

 inference that might be extended to all, could we show them to 

 be all cotemporaneous. That they are so is rendered probable by 

 their parallelism, and by the nearly uniform texture of the trap, 

 in whatever rock the dyke is found. The simple minerals too 

 contained in the trap favour this opinion. Calcareous and heavy 

 spar, and lime intimately mixed with the trap abound most in the 

 dykes of secondary rocks ; Iron pyrites is common to those both of 

 primitive and secondary districts ; and radiated zeolite, olivine and 

 augite are common to the dykes of primitive rocks and to the beds 

 of trap of secondary formation. 



From this inference, however, I must except a class of dykes 

 which run parallel to the metallic veins, and are probably inter- 

 sected by that class of dykes which I have been describing in the 

 present paper. An example of this new class of dykes is found at 

 Farland point in the county of Donegal, where alternating strata of 

 slaty quartz and sienite are traversed by a dyke of clay porphyry 

 bearing east of north 21° : a dyke of trap is found at the same place 

 bearing west of north 49°, the angle between the two being 100°. 

 There can be no doubt that they meet one another, but the spot 

 being covered by the sea, I could not discover the point of inter- 

 section. 



Whatever date and whatever agents we are disposed to assign to 

 the origin of dykes, their uniformly vertical and nearly parallel 

 positions evince that both they and the mountains which they 

 intersect have not undergone any modern disturbance beyond super- 

 ficial abrasion, but that they remain in the same situation as at the 

 remote period at which they were formed. 



