526 PROCEEDINGS OF THE AMERICAN ACADEMY. 



by a process of alteration, and therefore, according to him, all such 

 apatites are altered. The freshness of the Minot material makes such 

 a supposition inapplicable here. 



The axial ratio of the Minot apatite is the largest and the birefrin- 

 gence the lowest recorded for the species. It was interesting to see 

 what data existed for a comparison between fluorine or chlorine content 

 and the axial ratio, specific gravity, and birefringence. 



The normal angle c to x and also the specific gravity of apatite have 

 been held by numerous observers to diminish with increasing chlorine 

 content. 



G. Rose (Ref. 3 below), the first to discover the fluorine and chlorine 

 in apatite, stated as the result of his studies that the angles and specific 

 gravity were alike in all apatites of like composition, but that the reverse 

 of this statement was not proved. 



Von Kokscharovv (Ref. 2) extended this statement, holding that the 

 normal angle c to a; of all chlorine-containing apatite was somewhat 

 less than that of pure fluor-apatite. 



The analyses of Pusyrewsky (Ref. 18) seemed to confirm this view, 

 and he further maintained that the specific gravity regularly decreased 

 with increasing chlorine. 



Von Kokscharow (loc. cit.) incorporated the results of the last writer 

 with his measurements and published a table showing the relations of 

 the three values, but without comment. 



Baumhauer (Ref. 4) was the next to investigate the subject, and his 

 measurements, analyses, and specific gravity determinations seemed to 

 sujjport the supposed relations. His table has been republished by 

 several authors with slight modifications (Dana, Syst., 1892, 764, and 

 Weibull, Ref. 20 below).* 



In order more fully to test the matter a table has been prepared and 

 is given below, arranged according to increasing values of the angle 

 c to cc or of the axial ratio, and showing for all occurrences for which 

 accurate crystallographic data existed, the specific gravity, birefringence, 

 and chlorine and fluorine content so far as such data could be found. 



* In Baumliauer's table and in all later tables of the same kind the locality 

 Schlaggenwald is given with c to x 40° 20', the largest value for this angle ob- 

 served on apatite. Reference to tiie description of this occurrence by Schrauf 

 (Ref. 8 below) showed that the measured crystals were poorly adapted to measure- 

 ment, having curved faces ; and the average axial ratio calculated from all the 

 measurements gave a value much lower, about 40° 17'. This locality was tliere- 

 fore omitted from the table below. 



