DALY. 



AMPHIBOLES AND PYROXENES. 



321 



TABLE IV. 



"When the errors of 6 and 6" are unequal, the resulting error in p is 

 more significant and is most unfavorable when the errors of reading the 

 extinction in the two positions are of opposite sign. When equal 

 numerically, the error in p for actinolite and diopside is about four times 

 that made by directly reading the extinction on a plate cut parallel to 

 (010). The necessity for accurate measurement of 6' and 6" is evident. 

 In actual practice, moreover, several independent determinations of p 

 should be carried out and an average taken to secure the safest results. 



Two examples may suffice to show the kind of results obtained by 

 the author in the application of the method here outlined. The Philip- 

 stad hornblende already referred to was studied with reference to the 

 extinction on (010), first by the use of two carefully oriented sections 

 made by M. Werlein of Paris, then by the use of cleavage pieces. The 

 former gave an extinction on (010) of 15° 5' (white light). Extinction 

 was then read on a cleavage flake and found to be 20° 53', as the result 

 of averaging many readings. This flake was then turned into the posi- 

 tion where the plane perpendicular to the ray of light from the polarizer 

 made an angle of 77;^° with the plane of symmetry of the crystal. Ex- 

 tol. XXXIV. — 21 



