610 



VARIOUS FORMS OF 



[Ch. XXIX. 



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 seacoast, as represented in the annexed view 

 (fig. 678). In these instances, the greenstone of the dike, is usually 



Fig. 677. 



Fie. 678. 



immmmBm,^ 



Dike in a valley near Brazen Head, Madeira. 

 (From a drawing of Capt. Basil Hall, B. N.) 



Fissures left vacant by decomposed trap. 

 Strathaird, Skye. (MacCulloch.) 



Fig. 679. 



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 surround- 

 ing rocks. When this happens, two parallel walls of indurated strata 

 are seen protruding above the general level of the country and fol- 

 lowing 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. 679) by Dr. MacCulloch 

 represents part of a sea-cliff in Argyle- 

 shire, where an overlying mass of trap, 5, 

 sends out some veins which terminate 

 downwards. Another trap vein, a a, cuts 

 through both the limestone, c, and the trap, b. 



In fig. 680, 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 5 



Trap veins in Airdnarnurchan. 



