224 METASOMATIC PROCESSES IN A CASSITERITE VEIN, 



excess of the quartz, which is less abundant than in Slide 1), 

 Felspar is present in appreciable amount, showing replacement 

 by non-ferruginous mica, which seems to be the main metasomatic 

 constituent throughout. This replacement of felspar has occurred 

 chiefly, though not wholly, along the cleavage-planes. A littls 

 molybdenite, and some secondary haematite are present. Cavities 

 are to be seen in the quartz present on the vein-side of the slide, 

 but none are present in the quartz on the granite-side. Thus 

 the presence or absence of cavities serves to distinguish the vein- 

 quartz from that in the country-rock. On the side of the slide 

 approaching the granitic texture, a relic of graphic structure was 

 noted, which was distinct, though modified considerably by the 

 replacement of both the quartz and felspar by mica. 



Slide F corresponds with Slide G, save that some of the quartz 

 and felspar show replacement by mica. 



Slide G is of the normal granite. It is holocrystalline, the 

 grain-size is variable, and graphic structure is present. Quartz 

 is the most abundant constituent, after which albite and ortho- 

 clase are respectively the next most import:4,ot. The quartz and 

 orthoclase are frequently intergrown, giving rise to graphic 

 structure. (See Plate xx., fig. 1). A little biotite is present, 

 and also some secondary haematite. There is no fluorite present^ 

 nor is there any sign of the non-ferruginous mica, which is such 

 a characteristic feature of the vein-material. 



A series of four analyses was made, which though not quite com- 

 plete, yet casts much light on the origin of the vein. The analyses 

 A, C, and E are of those portions of the vein represented in text- 

 fig. 1; and analysis G is of the normal granite beyond the sphere 

 of metasomatic action. Analyses E^^ and are Gi quoted from 

 Lindgren,* the former representing the altered product of meta- 

 somatic action in a cassiterite vein from Altenberg and Zinn- 

 wald in Saxony, and the latter the normal granite from which 

 this alteration took place. The following are the analyses,, 

 lettered to correspond with the text, and with text-fig. 1. The 

 molecular ratios are also given. 



* Metasomatic Processes in Fissure-veins. 



