488 Bascom and Goldschmidt — Anhydrite Twin. 



constitutes the plane of contact for the two individuals. 

 p = 15 (151) is a new form for anhydrite. It appears on the 

 smaller individual with a well defined face and occupies the 

 place of the plane f, which, though present on the larger 

 individual, is missing on the smaller. 



The face gives a fair signal and the form was determined by 

 the following measurements : 



Measured: <fr> = 41° 57' : 16° 23' 

 Calculated : " = 41° 45' : 16° 42' 

 Not only is the agreement between the measured and calcu- 

 lated angles assuring, but the form fits perfectly into the series, 

 as is shown by the following discussion : 



n 



/ 



p* 



b 



12 



13 



15 



oo 



2 



3 



5 



GO 



p^ = 10 1 

 £=01 

 i(q-\) = — £ 1 2 oo 

 The series 6b is a normal one and the form j> is accordingly 

 established. 



General Discussion. — Anhydrite twins, with twinning plane 

 parallel to r (10), were first described by Hessenberg.f These 

 crystals came from Berchtesgaden and are an inch in diameter. 





a-'. Ic .V 







. — ~i 



o' .- T / '. 



o 



< 



1 





"---^. ' i z' 











^^\/ y^^ 













/ \^/ \ 











1 / - ' 



/ /\ 1 











\ / V 



/ ./ \ 1 







a 





' / v \ l 



/ / \l 

 / >/ \ J 



' ' / 1 





a 





V\ li 



c' 





• \ p / 



' \ / n- / 



i ^^ / 1 







V / 







1 y/ \^ 1 

 1 ^^^ \j 





< 





3 v/ .... --- 



It' 



0' 



3 





,'r> c | ;a' 





• — Crystl o-— Cryst.E. 



*Senkenb. Abh. 1872, vol. viii, p. 12, pi. 1, fig. 14. 



