612 



TRAP DIKES AND VEINS. 



[Ch. XXIX. 



wider after the lava on each side had consolidated, and the additional 

 melted matter poured into the middle space maj have cooled more 

 rapidly than that at the sides. 



In the ancient part of Vesuvius, called Somma, a thin band of 

 half-vitreous lava is found at the edge of some dikes. At the junc- 

 tion of greenstone dikes with limestone, a sahlband, or selvage, of 

 serpentine is occasionally observed. On the left shore of the fiord of 

 Christiania, in Norway, I examined, in company with Professor Keil- 

 hau, a remarkable dike of syenitic greenstone, which is traced through 

 Silurian strata, until at length, in the promontory of Nsesodden, it 

 enters mica-schist. Fig. 682 represents a ground plan, where the 

 dike appears 8 paces in width. In the middle it is highly crystalline 

 and granitiform, of a purplish color, and containing a few crystals of 

 mica, and strongly contrasted with the whitish mica-schist, between 

 which and the syenitic rock there is usually on each side a distinct 

 black band, 18 inches wide, of dark greenstone. When first seen, 

 these bands have the appearance of two accompanying dikes ; yet 

 they are, in fact, only the different form which the syenitic materials 

 have assumed where near to or in contact with the mica-schist. 

 At one point, a, one of the sahlbands terminates for a space ; but 

 near this there is a large detached block, b, having a gneiss-like struc- 

 ture, consisting of hornblende and felspar, which is included in the 

 midst of the dike. Eound this a smaller encircling zone is seen, of 

 dark basalt, or fine-grained greenstone, nearly corresponding to the 

 larger ones which border the dike, but only 1 inch wide. 



It seems, therefore, evident that the fragment, b, has acted on the 

 matter of the dike, probably by causing it to cool more rapidly, in 

 the same manner as the walls of the fissure have acted on a larger 

 scale. The facts, also, illustrate the facility with which a granitiform 

 syenite may pass into ordinary rocks of the volcanic family. 



Fig. 682. 



Syenitic greenstone dike of Nassoddcn 



Christiania. 



Fig. 



Green- Syenitic Green- 



stone, rock. stone. 



b. Imbedded fragment of crystalline schist 

 surrounded by a band of greenstone. 



Greenstone dike, with fragments of gneiss. 

 Sorgenfri, Christiania. 



The fact above alluded to, of a foreign fragment, such as b, fig. 

 682, included in the midst of the trap, as if torn off from some sub- 



