80 LA TOUCHE : GEOLOGY OF WESTERN RAJPUT ANA, 



connection with the surrounding magma, have a more or less well 

 denned, hexagonal outline, but in most cases their outlines are 

 rounded and they are evidently sections of inlets. 1 In one or two 

 cases the quartz crystals appear to have been partially melted and 

 drawn out into strings along the lines of flow (PI. IX, fig. 3). 



A very beautiful and interesting phenomenon is exhibited by the 

 quartz phenocrysts in several specimens, and is illustrated by figs. 4, 5 

 and 6, PI. IX. This consists in a growth of the crystal to a greater or 

 less distance into the surrounding groundmass, either during or sub- 

 sequent to the consolidation of the rock. In all cases the secondary 

 quartz surrounding the original phenocryst is in optical continuity 

 with it, and extinguishes simultaneously with it between crossed 

 Nicols. Sometimes the secondary quartz forms irregular patches, 

 looking like small tufts of cotton wool, only partially surrounding 

 the phenocryst. In this particular instance the hexagonal outline 

 of the original crystal is very distinct. In other cases these 

 patches are united into a continuous area surrounding the crystal 

 sometimes with irregular boundaries as in fig. 4, and sometimes 

 following the outlines of the crystal pretty closely as in fig. 5. 

 In one example the crystal appears to have been broken into two 

 halves, lying in a slightly different optical relation to the plane of 

 polarised light traversing them, so that they give slightly different 

 colours between crossed Nicols ; each portion has its concentric area 

 or " court " enclosing it, each polarising in the same colour as the 

 portion of the crystal it surrounds, and the two "courts " meet along 

 a line about midway between the two halves of the crystal. With a 

 high power the details of the structure of these " courts " can be easily 

 examined. The secondary quartz ramifies out from the edge of the 



1 It appears to me that the hexagonal inclusions of the groundmass may 

 also be sections of " inlets " that have corroded the crystal along zonal lines, and 

 that their presence does not afford a convincing argument for the contempora- 

 neous formation of the phenocrysts, as suggested by Zirkel (Rosenbusch. Mikr. 

 Phys. Massig. Gest , pp. 52, 99J. 



( So ) 



