L04 



LYELL'S ELEMENTS OF GEOLOGY. 



Trap Dikes and Veins. 



Fig. 90. 



In the islands of Arran, Sky, and other parts of Scotland, 

 where sandstone, conglomerate, and other hard rocks are tra- 

 versed 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. 90.) In these instances 



the greenstone of the dike is usu- 

 ally more tough and hard than the 

 sandstone; but chemical action, 

 and chiefly the oxidation of the 

 iron, has given rise to the more ra- 

 pid 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 indurated strata are 



Fig. 91. 



Fissures left vacant by decomposed trap, seen protruding above the general 

 Strathaird, Sky. (MacCuiioch.) 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 bifur- 

 cating and ramifying, and sometimes they are so tortuous as to 

 be called veins, though this is more common in granite than in 

 trap. The accompanying sketch 

 (Fig. 91.) by Dr. MacCuiioch repre- 

 sents part of a sea-cliff in Argyleshire, 

 where an overlying mass of trap, 6, 

 sends out some veins which terminate 

 downwards. Another trap vein, a a, 

 cuts through both the limestone, c, 

 and the trap, b. 



In Fig. 92. 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 from five to seven feet in width, which will 

 afford a scale of measurement for the whole. 



In the Hebrides and other countries the same masses of trap 



Trap veins in Airdnamurchan. 



