451 



Vesicular A term applied to cellular volcanic rocks, when the holes are few 

 and small. 



Viridite A term used to designate the green decomposition-products, 

 characteristic of the altered basic rocks, which do not admit of exact 

 determination. 



Vitreous Of the nature of glass. 



Vitrophyric A term applied to those quartz-porphyries whose groundmass is 

 composed essentially of glassy matter, with or without microlites and 

 crystallites. 



Volcanic A term used in petrography, in contradistinction to plutonic, to 

 designate such rocks as have consolidated at or near the surface. Syn. 

 Effusive. 



Weathering" (Ger. Verwitterung) The changes in the substance of a rock 

 produced by surface agencies. 



XenomorphlC See Allotriomorphic. 



Zone In crystallography, a group of faces which are parallel to one and the 

 same straight line (zone-axis). Such faces are said to be tauto zonal. A 

 plane perpendicular to the zone-axis is the zone-plane. In stratigraphy, a 

 bed or series of beds characterized by an assemblage of fossils of which 

 one species is taken as the index ; as, for example, the ammonite-zones of 

 the Lias. 



Zoned, Zonal A structure exhibited by felspar and other minerals in which 

 the growth of the crystals is marked by bands or zones of inclusions or 

 a change in composition (Ger. Schalenbau, isomorphe Schichtung). 

 Another case of zoning is when one mineral is completely surrounded by 

 a narrow band or zone of a second. 



Zoogenic Of animal origin, as, for instance, the coral reefs. 



FINIS. 



