14 Notes on the Diabase Rocks 



OP (001). 



oo P oo (100). 



oo P oo (010). 



3° 30' 



21° 0' . 



13° 0' 



5° 0' 



28° 0' . 



10° 30' 



5° 30' 



28° 0' . 



9° 30' 



5° 30' 



22° 30' . 



7° 30' 



6° 0' 



26° 0' . 



10° 0' 



7° or 



25° 0' 



12° 0' 



The investigations of Des Cloizeaux, and more recently 

 those of Schuster,* show that the angles formed by the 



plane of vibration with the edge OP — oo P oo depend upon 

 the position of the optic axial plane as well as upon the 

 position in that plane of the optic axes themselves. As 

 the last-quoted authority well puts it — "the plane of vibra- 

 tion moves round the edge PM." 



It seems evident that the differences of position of the 

 optic axial plane in the triclinic felspars is connected with 

 variations in chemical composition according as the felspars 

 contain, for instance, Na 3 O, K 2 O, CaO, singly or together, in 

 variable proportions. ■(* 



According to Schuster's late observations the optical angles 

 in oligoclase, andesine, and labradorite are as follows : — 



OP (001). oo"P"oo (100). oo P oo (010). 



+ 2°to+l° ... +18° ... +3° to + 2° 



- 1° to - 2° - 4° to - 6° 



_ 40 t0 _ 50 _ _ 30 o _ _ 17 o 



Taking, therefore, in the list of measurements which I have 

 given from these plagioclase felspars, the lowest for OP and 



— u 



the highest for 00 P 00 and 00 P 00 respectively, we have 



the following : — 



OP (001). 00 P 00 (100). 00 P 00 (010). 



3° 30' ... 28° ... 13° 



On comparing this series of measurements with those quoted 

 from Schuster, it suggests that the felspars to which they 

 refer stand between a normal andesine and labradorite. 

 But at the same time it must be borne in mind that, owing 

 to the difficulties in the way of obtaining perfectly accurate 



* tfber die ojrtische Orientirung der PlagioMase, Sitzungslerichte der 

 Wiener Akademie, Vol. lxxx., p. 1, July, 1879 j also, Neues Jahrbuch der 

 Mineralogie, 1880, Vol. ii., pt. ], p. 8. 



f Also, Ba O and, perhaps, Sr O. 



