4 



ON COTEMPORANEOUS VEINS. 



large. It contains cotemporaneous veins of quartz ; 

 also of mica ; and of a granular rock composed of 

 quartz and mica, differing from mica-slate in 

 wanting the slaty structure ; thus bearing the 

 same relation to mica-slate, that granitic gneiss 

 does to common gneiss. Greenstone, a well known 

 aggregate rock of hornblende and felspar, contains 

 numerous cotemporaneous veins of felspar, also 

 of hornblende, and of greenstone. These veins 

 are sometimes of great extent : in the floetz-green- 

 stone of Salisbury Craigs near Edinburgh, there 

 are cotemporaneous veins of felspar, upwards of 

 an hundred feet long, and from half an inch to 

 two inches wide. 



All the cotemporaneous veins we have just de- 

 scribed, contain the same minerals as those that 

 compose the rock which they traverse. It fre- 

 quently happens, however, that cotemporane- 

 ous veins occur composed of minerals that 

 differ considerably from those of the rock in 

 which they are contained : Thus serpentine con- 

 tains cotemporaneous veins of asbest, talc, steatite, 

 and lithomarge : these substances, it is true, have 

 a strong oryctognostic affinity with serpentine, but 

 differ more from it than granular gneiss does from 

 common gneiss, or granular mica- slate from com- 

 mon mica-slate. Flinty-slate is frequently tra- 

 versed by numerous cotemporaneous veins of 

 quartz ; and transition-limestone with numerous 

 ' veins of calc-spar. Numerous examples also oc- 

 cur of cotemporaneous veins filled with materials 



