138 HOLLAND: CHARNOCKITE SERIES. 



The shapeless crystals of quartz are often crowded with 



minute hair-like inclusions, which are arranged 



Adcuiar indusions in the with cryst allographic regularity. The crys- 



tallographic disposition of two sets of these 

 acicular inclusions are easily studied in sections cut at right angles 

 to the vertical axis (isotropic sections). In such sections (No. 1604, 

 for instance) long acicular inclusions lying in the plane of section (that 

 is, parallel to the basal plane) cross one another at angles of 6o° 

 and show straight extinction. The angles formed by the crossing 

 of these long needles are bisected by a set of shorter needles lying 

 oblique to the plane of section. Taking the first set of inclusions to 

 be arranged parallel to the lateral axes, that is, to the lines joining 

 the opposite solid angles of the prism of the 1st order, the second 

 set must lie in the secondary set of symmetral planes, and, being 

 oblique to the basal section, probably lie parallel to some pyramidal 

 face. The numerous black dots seen in the basal sections represent 

 the cut ends of those which in vertical sections are found to lie 

 parallel to the axis of minimum optical elasticity, and, therefore, also 

 parallel to the vertical axis. The three sets of inclusions exhibited 

 by basal sections of blue quartz are shown in the diagram (fig 2), 



(1010) m> 



Fig. 2. -^Diagrammatic representation of the needles in basal sections of Quarts. 

 On comparing the phenomena presented by the basal sections 

 with those exhibited by longitudinal ones, the assumed positions for 

 two of the sets of needles are easily verified at once. Long 



( 20 > 



