■232 METASOMATIC PROCESSES IN A CASSITERITE VEIN. 



manner in which he has placed every facility at my disposal in 

 ■the carrying out of this investigation. 



EXPLANAnON OF PLATES XVIII. -XX. 



Plate xviii. 

 Section across the fissure-vein, showing normal granite on each side, and 

 vein-material between. 



Plate xix. 

 Fig. 1.— Section showing cavities, and negative crystals in vein-filling; ( x70). 

 Fig.2.— Quartz showing replacement by non-ferruginous mica. Crossed 



nicols; ( x 20). 

 Fig.3.— Showing fluorite in centre, and biotite replaced by non-ferruginous 



mica; ( x 20). 

 iFig.4.— Albite crystal, showing replacement by non-ferruginous mica; 



( X 20). 



Plate XX. 



Fig. 1.— The normal granite, with graphic structure. Crossed nicols; ( x 20) 

 Fig.2. — Replacement of quartz by non-ferruginous mica; ( x 20). 

 Fig. 3. —Biotite, with replacement by non-ferruginous mica; ( x 20). 

 ^ig.4. — Quartz, showing replacement by non-ferruginous mica along 

 cracks; ( x 20). 



