Ch. XXIX.] 



TEAP DIKES AND VEINS. 



477 



Fig. 625. 



case the dike stands prominently out in the face of precipices, or on the 

 level surface of a country. 



In the islands of Arran and Skye, and in other parts of Scotland, where 

 sandstone, conglomerate, and other hard rocks are traversed by dikes 

 of trap, the converse of the above phenomenon is seen. The dike, 

 having decomposed more rapidly than the containing rock, has once 

 more left open the original fissure, often for a distance of many yards 

 inland from the sea-coast, as represented 

 in the annexed view (fig. 625). In these 

 instances, the greenstone of the dike is 

 usually more tough and hard than the 

 sandstone ; but chemical action, and 

 chiefly the oxidation of the iron, has 

 given rise to the more rapid decay. 



There is yet another case, by no means 

 uncommon in Arran and other parts of 

 Scotland, where the strata in contact with 

 the dike, and for a certain distance from 

 it, have been hardened, so as to resist the 

 action of the weather more than the dike 

 itself, or the surrounding rocks. When 

 this happens, two parallel walls of indu- 

 rated strata are seen protruding above 

 the general level of the country and following the course of the dike. 



As fissures sometimes send off branches, or divide into two or more 

 fissures of equal size, so also we find trap dikes bifurcating and ramifying, 

 and sometimes they are so tortuous as to be 

 called veins, though this is more common 

 in granite than in trap. The accompany- 

 ing sketch (fig. 626) by Dr. MacCulloch 

 represents part of a sea-cliff in Argyleshire, 

 where an overlying mass of trap, b, sends 

 out some veins which terminate downwards. 

 Another trap vein, a a, cuts through both 

 the limestone, c, and the trap, b. 



In fig. 627, a ground plan is given of a 

 ramifying dike of greenstone, which I observed cutting through sandstone 

 on the beach near Kildonan Castle, in Arran. The larger branch varies 





Fissures left vacant by decomposed 

 trap. Strathaird, Skye. (MacCul- 

 loch.) 



Fig. 626. 



Trap veins in Airdnamurchan. 



Fig. 627. 



Ground plan of greenstone dike traversing sandstone. Arran. 



