Pirsson and Washington — Geology of Red Hill, JV. II. 439 



with the low birefringence makes it impossible to deal further 

 with the arrangement of the optic system. The angle cone 

 •is about 14°; this is different from most amphiboles of this 

 group where a is nearer c. The plane of the optic axes, so far 

 as one may safely judge, lies in the symmetry plane. 



Since the only minerals in the rock outside of the quartz 

 and alkalic feldspar which is present in any amount, beyond 

 mere traces, is this hornblende, its composition can be pretty 

 closely calculated from Washington's chemical analysis of the 

 rock given beyond. This shows it to be as follows : 





I 



II 



III 



IV 



V 



VI 



Si0 2 ... 



45-4 



49-65 



45-53 



34-18 



0-756 



0-827 



TiO„ . . . 

 A1.0," 



Fe 2 3 -_ 



4-0 







1-52 

 11-52 

 12-62 



•050 





1-34 

 17-66 



4-10 

 9-35 



0-013 

 •110 



16-8 



•105 



FeO ... 



22-0 



1955 



23-72 



21-98 



•306 



•271 



M<?0_._ 



CaO ... 



0-6 

 4-2 





2-46 

 4-89 



1.35 



9-87 



•015 

 •075 





3-16 



•056 



Na 2 .. 



6-7 



761 



6-07 



3-29 



•108 



•122 



Rest . . . 





1-67 



0-88 



3-26 











Total 



99-7 100-64 99-96 99-59 



I. Amphibole from Red Hill. 

 II. Riebeckite from granite, Quan. Class. Ign. Rocks, 1903, 

 Table xiii. 



III. Cataphorite, San Miguel, Azores. Rosenb., El. Gest., 

 p. 266, 1898. 



IV. Hastingsite, Hastings, Ontario. Adams, this Jour. vol. i, 

 1896, p. 213. 



V. Molecular ratios of No. 1. 

 VI. Molcular ratios of No. 2. 



It is therefore very close in composition to the riebeckite 

 from the Quincy granite described by Washington,* and as 

 may be seen by reference to the other analyses, is similar to 

 alkalic hornblendes in general. Both Nos. I and II have the 



ill 



general composition m NaFeSi 2 6 . n FeSi0 3 , as may be 

 readily shown from the molecular ratios in the following 

 manner : 



No. I. 



Si0 2 0-756 ] 



Ti0 2 .... 



•050 



A1 2 3 ..._ 





Fe 2 3 ... 



•105 



FeO 



•306 



Mg 



•015 



CaO 



•075 



Na,0_.._ 



•108 



806 



■j -420 = 4-00 

 0-97 



•386 



•105 -105 = 1-00 



•396 -396 



1-00 



•108 -108 = 1-03 

 * This Journal, vol. vi, p. 



No. II. 



827 -827 



j 492 

 335 



4-02 

 1-02 



013 

 110 

 271 ) 



056 ) 



123 -123 = 1-00 



— V -327 



122 -122 

 181, 1898. 



•327 = TOO 

 •122 = 0-99 



