Ford and Tillotson — Orthoclase Twins. 151 



defined, as the former is coated with a thin film of green gegirite, 

 while the latter is clean. 



The crystal represented in figure 4, although in general of 

 the same type, differs in that the individual shown in normal 

 position is apparently set into the other, the latter in a measure 

 surrounding the former. In order to ascertain the relations 

 existing between the two individuals of this twin, a section was 

 cut through the crystal along a plane at right angles to the c 

 and o faces and close to the front of the crystal. Figure 5 

 shows the relationship between the two individuals along this 

 plane as indicated by the section, the shaded portion of the 

 figure representing that part of the crystal, placed in twin posi- 

 tion. Where the dividing line was straight and clean cut, 

 there was a slight crack to be observed between the two parts 

 of the twin, but elsewhere there was considerable kaolinization 

 of the feldspar and this line became more irregular and indis- 

 tinct. 



The crystals were investigated both optically and chemically, 

 in order to ascertain if they showed in these, respects any un- 

 usual features which might be connected with their peculiar 

 development. Sections were made parallel to the three pina- 

 coids, a (100), I (010) and c (001). They showed that the min- 

 eral was slightly kaolinized, but not to any great extent. There 

 was no evidence of microscopic twinning or of parallel growth 

 of more than one feldspar. The optical orientation is that 

 most common with orthoclase, the axial plane being perpendi- 

 cular to o (010), a inclined to the a axis +4° 54 r , and p ^>v. The 

 conformity in its optical properties to normal orthoclase is 

 rather surprising when the large amount of soda shown to 

 be present by the analysis is considered. The results of the 

 analysis by Tillotson follow : 



Si0 2 __ 64-01- 



A1 2 3 20-19 



K 2 10-48 



Na 2 5-37 



Total 100-05 



This particular occurrence of the tinguaite rock was not ana- 

 lyzed because of the alteration it had undergone through weath- 

 ering, but a closely similar and fresh rock from Cone Butte was 

 analyzed by Pirsson* and the percentage of the alkalies found 

 by him agree closely with those of the present analysis. In 

 the Cone Butte tinguaite, however, the soda was considered to 

 belong entirely to either albite or nephelite, but in the West 



* This Journal, ii, 192, 1896. 



