ON COTEMPORANEOUS VEINS. 



5 



entirely different from the rock in which they are 

 situated : Thus, clay-slate is sometimes traversed 

 by cotemporaneous veins of quartz : serpentine, by 

 cotemporaneous veins of magnetic ironstone, and 

 clay ironstone, hy numerous cotemporaneous veins 

 of calc-spar, and also of mineral pitch. 



It is worthy of particular remark, that cotem- 

 poraneous veins sometimes open at the upper or 

 under side of the stratum or bed in which they 

 are contained, owing to the rent accidentally reach- 

 ing either to the upper or under surface. In- 

 stances of this appearance, are to be seen in the 

 greenstone of Salisbury Craigs, where cotempo- 

 raneous veins of felspar traverse greenstone, but? 

 open in some places at the lower side of the bed 

 where it rests upon sandstone. Very striking ap- 

 pearances of the same kind occur in Glencloy, in 

 the island of Arran : there the cotemporaneous 

 veins are of greenstone, and above a foot wide ; 

 they traverse a bed of greenstone, and open on 

 its lower side where it rests upon sandstone. Vide 

 PL I. Fig. i. These veins, where in contact 

 with the sandstone, as at a, are sometimes partly 

 intermixed with it. Similar appearances occur 

 in other rocks, as mica-slate, transition-lime- 

 stone, and gneiss. Thus we often observe in 

 mountains composed of mica-slate, cotempo- 

 raneous veins of quartz opening on the upper 

 and under sides of a stratum, which rests upon 

 and is., covered by ... strata of mica-slate; but 



A 3 



