368 Schaller — Siderite and JBarite from Maryland. 



perfectly plane, with the result that each face gave several 

 signals. 



After this value for c was obtained and found to be differ- 

 ent from the commonly accepted value, 10 more crystals were 

 measured and after the greatest care in so adjusting each crys- 

 tal that the reflections from the scalenohedral faces of v fell as 

 nearly as possible in a straight line, each reflection was care- 

 fully measured. In the case of more than one signal, the ex- 

 tremes were measured and the average taken. The average 

 value for the p angle for each crystal (six faces) is : 



68 c 



M4' 



21' 



26 



27 



18 



20 



15 



19 



16 



29 



Av. 68°204/, .-. c = -8243 



The values for the two extremes, 68° 14' and 68° 29', are : 



c = -8197 and e = -8302 



Taking the average of the values found from r, v, y, namely, 

 •8240, and that found from the 10 crystals, namely, '8243, we 

 get as a value for the c-axis for siderite of known purity, 



c = '8241 



As however, this value differs considerably from that adopted 

 for siderite and as the crystal faces were at times uneven and 

 the angular measurements showed considerable variation, 

 the writer is rather hesitant in urging this new value. It, at 

 least, serves to throw some question over the commonly accepted 

 value and shows the need of additional measurement of material 

 shown by chemical analysis to be pure. 



The complete lists of forms so far observed on siderite is as 

 follows : 



c = 0001 



*f = 



5052 



d = 



0881 



m — 1010 



M = 



4041 



8 = 



4486 



a = 1120 



e = 



0112 



a = 



4483 



g*-— 1012 



h* = 



0332 



V = 



2131 



z*= 3034 



f = 



0221 



P = 



2461 



r = 1011 



O) =: 



0773 



y\ = 



3251 



l\= 1015 



6' = 



0551 



»}■= 



4159 



Those with no reference mark are found in Dana's Miner- 

 alogy. 



*Gonnard, Bull. Soc. Min., xviii, 382, 1895 ; also 1st supplement, Dana. 



f The present paper. 



jCesaro, Ann. Soc. G. Belg., xviii, 1891 ; also 1st supplement Dana. 



