25S 



Tlie results of chemical analysis were checked, a? far as possible, 

 by measurements of the physical and optical properties, with 

 very concordant results. 



Crystallographic Measurements.* 



These measurements of the angle between the (001) and (010) 

 cleavages were made by means of a Miers' goniometer reading to 

 one minute. The cleavajoe flakes give fair to good images. The 

 maximum value obtained was 89° 56', the minimum 89° 45' with 

 an average of 89° 53' for nine readings. 



Optical Measurement. — No diamonds being available for 

 sectioning, this part of the work was postponed as long as 

 possible in the hope of their arrival. When this was finally 

 despaired of and the work commenced everything had to be done 

 in a short time. Small fragments of the felspar were broken ofif • 

 and pieces with two good cleavages selected. These were then 

 broken parallel to one cleavage and the two parts ground down 

 on different cleavages and mounted. No oriented sections other 

 than these could be cut. The extinction angles were carefully 

 measured. No instrument was available for the measuring of the 

 axial angle. The sections were examined under convergent light 

 with a Swift's 1" NA 0*97 objective. 



Most of the sections exhibit excessively fine multiple twinning. 

 Those parallel to (001) show striations after both albite and 

 pericline laws, those parallel to (010) exhibit pericline lamellae 

 only. The fineness of the lamellae, sinking to submicroscopic 

 dimensions, makes optical measurements very difficult. As the 

 mean of a large number of readings the following results were 

 obtained : — 



Extinction on basal plane, measured from the trace of (010), 

 varies from 2^ to 5°. Extinction on (010), measured from the 

 trace of (001) is 8° in the positive sense. Sections parallel to 

 (010) give a well defined figure in convergent light, practically 

 centrally situated in the field of the microscope the optical sign 

 is negative. Dispersion is noticeable, p > v. 



Specific gravity of the Felspar. 



( 1 ) By means of Klein^s solution — The indicators used were 



Goldschmidt's Natural Indicators by F. Krantz of Bonn. The 



particles were found to nearly coincide in density with 



cordierite 2-579, some were a little lighter and some a little 



heavier, but none reaching the next values 2*566 and 2.591. The 

 density might therefore be taken as 2-58. 



*The phenocrysts of felspar are fairly idiomorphic, but it is difficult to 

 obtain isolated crystals sufficiently perfect tor external measurements. — 



