230 METASOMATIC PROCESSES IN A CASSITERITE VEIN, 



An excellent example of No. 2 is given in Plate xix., fig. 4, 

 which illustrates the albite crystal described from Slide D. Com- 

 plete crystals of mica are present in the felspar without discon- 

 tinuity of their surfaces. 



Plate XX., fig. 3, illustrates No.3. Here biotite has been 

 replaced by the paragonite-sericite mica, and several pieces of the 

 former are completely surrounded by the latter. Almost any 

 random section will serve to illustrate how the replacement has 

 taken place along cracks and fissures, thus illustrating No A. An 

 example of this is afforded by Plate xx., fig. 4. 



Mr. Emmonds has suggested the following field-structures as 

 necessary criteria — 



(1) "Absence of symmetrical banding or comb-structures in 

 the vein-material, and of breccias of country-rock cemented by 

 vein-material." 



(2) " Great irregularity in width of the ore-bodies, which may 

 reach very great dimensions." 



(3) " General lack of definition between ore-body and wall- 

 rock." 



Though, in the microscopical examination, it was noted that 

 the cavities in the primary vein-quartz had a tendency to arrange- 

 ment in planes parallel to the fissure-plane, yet in no sense could 

 it be said that symmetrical banding was present. Field-observa- 

 tions failed to detect either the slightest trace of such banding, 

 or of breccias of country-rock included in vein-material. 



The irregularity in width of the vein examined has been 

 already mentioned. The fact that there is a progressive change 

 from ore-body into country-rock shows that there is a lack of 

 definition between the two. Thus the chain of evidence for 

 metasomatism seems complete. 



Summary — A sample constituting a complete transverse section 

 of a cassiterite vein from near Inverell in New England was 

 selected for examination. The object was to investigate the 

 processes involved in the formation of such veins, these processes 

 iiaving been generally described as being of a metasomatic 



