Ch. XXIX.] 



TRAP DIKES AND VEINS. 



479 



Fig. 629. 



Syenitic greenstone dike of Ncesodden, 

 Cbristiania. 



Green- 

 stone. 



b. Imbedded frngment of crystalline schist sur- 

 rounded by a band of greenstwne. 



enters mica-scliist. Fig. 629 rep- 

 resents 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 wdiitish 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, 6, having a gneiss-like structure, consisting of hornblende and 

 felspar, which is included in the midst of the dike. Round 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 

 one inch wide. 



It seems therefore evident that the fragment, 5, has acted on the mat- 

 ter 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. 



The fact above alluded to, of a foreign fragment, such as 5, fig. 629, 

 included in the midst of the trap, 

 as if torn off from some subjacent 

 rock or the walls of a fissure, is 

 by no means uncommon. A fine 

 example is seen in another dike 

 of greenstone, 10 feet wide, in 

 the northern suburbs of Chris- 

 tiania, in Norway, of which the 

 annexed figure is a ground plan. 

 The dike passes through shale, 

 known by its fossils to belong- 

 to the Silurian series. In the 

 black base of greenstone are angular and roundish pieces of gneiss, some 

 white, others of a light flesh-color, some without lamination, like granite, 

 others with laminae, which, by their various and often opposite direc- 

 tions, show that they have been scattered at randon through the 

 matrix. These imbedded pieces of gneiss measure from 1 to about 8 

 inches in diameter. 



Mocks altered hy volcanic dikes. — After these remarks on the form 



Greenstone dike, with fragments of gneiss. 

 Sorgenfria, Christiania. 



