DESCRIPTION OF THE CHIEF TYPES. I3g 



needles lying in the plane of the vertical sections are found on 

 application of the quartz wedge to be parallel to the axis of minimum 

 optical elasticity, which is the vertical crystallographic axis also. 

 These are the needles whose cut ends appear as mere dots in the 

 basal sections. Others are found lying at right angles to the vertical 

 axis, some in the plane of section and some oblique to it ; these 

 clearly represent the needles lying parallel to the basal plane and 

 assumed to be also parallel to the lateral crystallographic axes. 

 Besides these there are others lying oblique to the directions of ex- 

 tinction and oblique also to the plane of section. To verify the posi- 

 tion of these needles, which are assumed to lie parallel to the rhom- 

 bohedron, it has been necessary to find a section cut parallel to the 

 prism. Such a section would be one in which the polarisation colours 

 attain a maximum order, and in which long needles should lie in the 

 plane of section, both parallel and perpendicular to the vertical crystal- 

 lographic axis, whose direction is determined by means of the quartz 

 wedge. Such a case occurs in section No. 1781. In this section it 

 was found that besides numerous needles lying oblique to the plane 

 of the section there were other sets lying in the plane of the section— 

 (1) one set parallel to the vertical axis, (2) another set parallel to the 

 basal plane, and (3) a set meeting the vertical axis at an angle of 52 

 and meeting the trace of the basal plane at angle of 38 , agreeing thus 

 with the inclinations of the unit rhombohedron in quartz. With the 

 rough means supplied by the cross-wires of a microscope, and in the 

 presence of many other possible planes approximating to the rhombo- 

 hedron it is dangerous of course to put too much reliance on this 

 determination ; but it is worth recording that the determinations made 

 give results which would be expected if it were known that the 

 needles were lying parallel to the face of the unit rhombohedron. 



I conclude, therefore, that the hair-like inclusions, to which prob- 

 ably the blue colour of the quartz is due, are arranged with crystal* 

 lographic regularity as follows :•— 



(a) Parallel to the lateral axes, and thus lying in the prin- 

 cipal planes of symmetry. 



( 21 ) 



