PALACHE. — CRYSTALLOGRAPHY OK LEADHILLITE 451 



which in twin position by either law lies about six degrees from the 

 direction of the clinodome zone. Here, however, the polar distances 

 of the faces in the two zones are different, and the result of the 

 twinning is generally the formation of wedge-shaped faces dovetail- 

 ing irregularly into one another (Figure 8). 



It will be seen from what has been said that the twinning does 

 not in any way obscure, but rather tends to increase the pseudo-rhom- 

 bohedral appearance of the crystals. Figure 9 is intended to bring out 

 this striking habit. 



Cleavage plates examined under the microscope in polarized light 

 are usually found to be twins of the second kind mentioned, but in 

 thin plates the lamellae appear to be united on the prism m. When 

 a sufficiently thick plate is examined, the lamellae are seen to be 

 oblique to the cleavage, and the composition face was found to be 

 parallel to v (T22). Twins of the third kind, in polarized light, 

 usually show three sets of axial figures inclined to each other at 60° 

 and they do not give complete extinction in any position. 



No chemical analysis was made of this leadhillite, and the optical 

 characters have been only partially determined. The axial angle of 

 a cleavage plate was measured in air and in cedar oil with the 

 following results : 



2E Na = 19° 54' 2E Li = 19° 14' Temp. 23° C. 



2H Na = 13° 24' 2H Li = 12° 38' (in cedar oil) 



The axial angle was observed to grow smaller with increase of tem- 

 perature, but no successful measurement of the rate of change, nor 

 of the temperature at which it becomes uniaxial, was obtained. 



This study was begun at the time of the receipt of the leadhillite, 

 by Palache, but the crystals proved so complex that it was thought 

 best to put the matter aside in the hope that more material would 

 be found for study without breaking up any of the original lot. 

 Several years elapsed, and the investigation was renewed by La Forge, 

 when, by using a part of the finer specimens, material was obtained 

 which sufficed to unravel the complexities of the crystallization. The 

 work was again interrupted by the illness of the last named, and again 

 a long period passed before the results obtained could be put into 

 shape for publication. In its present form the paper has been prepared 

 by Palache, but the observations in large part, and all of the calcu- 

 lations involved, as well as the drawings, are the work of La Forge. 



