H. L. Wells and S. L. Penfield — Gerhardtite, etc. 51 



in fitf. 1. There are two cleavages, which serve for orientation, 

 one basal, parallel to e, as perfect as the most perfect cleavage 



of gypsum, a second, less perfect, par- 

 allel to the macropinacoid. The crys- 

 tals can be readily bent, in which case 

 they crack and separate along the lat- 

 ter direction. The most prominent 

 forms on the crystals, besides the basal 

 plane, are a series of pyramids occur- 

 ring in oscillatory combination, which 

 makes their indentification somewhat difficult. The best meas- 

 urements were obtained from a small but very perfect macro- 

 dome which was found on two crystals. Owing to the frag- 

 mentary nature of the crystals and the difficulty of identifying 

 the pyramidal planes, their orthorhombic form might be 

 doubted were it not for their optical properties. 



The axial ratio was obtained from the following measure- 

 ments: 



Ca2 001a201 = 

 z~p 201a111 = 



68° 16' 

 39° 3' 30" 



giving 5:6: c = 0-92175: 1: 1-1562 



The following forms were observed : 



P, 



001, 

 201, 

 110, 

 551, 

 221, 

 111, 







2-1 

 I 

 5 



t, 778, ,1 

 u, 334, f 

 v, 7-7-10, & 

 w, 223,' §• 

 x, 13-13-20, kv 



y, us, i 



The following is the table of measured and calculated angles, 

 the measurements being made on eight crystals, the number of 

 times each form was measured being given. 







Calculated. 



Measured. 



No. of times, 



m a m 



110a 110 



85° 20' 







Z a Z 



201 a 201 



43° 28' 



43° 34' 



1 



c a m 



001a 110 



90° 



90° l5'-90° 25' 



2 



c A.r 



001 a 551 



83° 19' 



83° 1' 



1 



C aS 



001a 221 



73° 40' 



73° 53' 



1 



C aJ) 



001 A 111 



59° 37' 



59° 23'-59° 57' 



6 



c^t 



001 a 778 



56° 11' 



55° 57'-56° 19' 



3 



C y,U 



001 a 334 



51° 59' 



51° 52'-52° 20' 



2 



C^V 



001 a 7710 



50° 3' 



49° 46'-50° 38' 



3 



c ~w 



001 a 223 



48° 40' 



48° 8'-49° 12' 



8 



C a X 



001 a 13-13-20 



47° 57' 



47° ll'-47° 56' 



5 



c^y 



001a 112 



40° 28' 



40° 13'-40° 18' 



2 



2aI 



13-13-20 a 1313-20 



60° 27' 



60° 9' 



1 



Only distinct reflections were recorded, though other forms 

 seemed to be present but were not definite enough to be deter- 

 mined. The variation in the measurements is large and may 

 be due in part to an accidental bending of the crystals. The 



