Ch. XXIX.] 



TRAP DIKES AND VEINS. 



479 



Green- 

 stone. 



b. Imbedded fragment of crystalline schist sur- 

 rounded by a band of greenstene. 



enters mica-schist. Fig. 629 rep- Fi s- 629 - 



j i i ,i Syenitic greenstone dike of Naesodden, 



resents a ground plan, where the ' s christiania. 



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 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, b, 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 b, 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 laminse, 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. 



Hocks altered by volcanic dikes. — After these remarks on the form 



Greenstone dike, with fragments of gneiss. 

 Sorgenfria, Christiania. 



