106 LYELL'S ELEMENTS OF GEOLOGY. 



Foreign Fragments in Dikes. 



Fig. 94. mica-schist. Fig. 94. repre- 



Sy enitic greenstone dike of Jsrasodden,Christiania. sentS a grOUnd plan, where 



the dike appears eight paces 

 in width. In the middle it is 

 highly crystalline and gra- 

 nitiform, of a purplish co- 

 lour, and containing a few 

 crystals of mica, and strong- 

 ly contrasted with the whitish 

 mica-schist, hetween which 

 and the syenitic rock there is 

 usually on each side a dis- 

 tinct 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 termi- 

 nates for a space ; but near this there is a large detached block 

 &, 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.* 



The fact above alluded to, of a foreign fragment, such as h 

 (Fig. 94.), included in the midst of thd' trap, as if torn off from 

 some subjacent rock or the walls of a fissure, is by no means 



uncommon. A fine illus- 

 tration is seen in a dike of 

 greenstone, ten feet wide, 

 in the northern suburbs of 

 Christiania, in Norway, of 

 which the annexed figure 

 is a ground plan. The 

 dike passes through shale, 

 known by its fossils to be- 

 long to the transition, or 

 Silurian series. In the 

 black base of greenstone 

 are angular and roundish 

 pieces of gneiss, some white, others of a light flesh-colour, some 

 without lamination, like granite, others with laminae, which, by 

 their various and often opposite directions., show that they have 



Greenstone dike, with fragments of gneiss , 

 Sorgenfri, Christiania. 



* This dike has been described by Professor Keilhau, of Christiania, in whose 

 company I examined it. 



