184 //. 8. Washington — Solvshergite and Tinguaite. 



One case of Carlsbad twinning was seen, and here and there 

 a microperthitic structure was very evident. The small feld- 

 spars of the groundmass are slender needles — probably of 

 anorthoclase — elongated (as shown by use of the quartz wedge), 

 parallel to a. The extinction is either parallel or at very small 

 angles. 



A few very small, colorless crystals of high refractive index 

 and double refraction, are supposed to be zircon. None of the 

 sodalite spoken of by Sears could be seen, though the analysis 

 shows that some of it is present. Magnetite is entirely absent. 



The clear, colorless, micro-groundmass in which the above 

 minerals lie is noncrystalline, and composed of nepheline and 

 analcite. These form irregular areas of varying sizes, passing 

 rather indefinitely from one into the other. Those of nephe- 

 line are distinguished by their faint double refraction, lack of 

 cleavage, and by the fact that the index of refraction is, as 

 shown by Becke's method, rather higher than that of the anor- 

 thoclase needles. One or two fair-sized crystals of nepheline 

 were observed. The patches of analcite are readily dis- 

 tinguished by their cubic cleavage, exhibited by well-defined 

 straight cracks crossing at right angles, by their generally iso- 

 tropic character, and by the fact that their refractive index is 

 notably lower than that of the feldspars. In places they show 

 a very faint double refraction, analogous to that of leucite, but 

 not as well marked. While the aegirite and feldspar needles 

 are scattered through both, yet the analcite areas are rather 

 poorer in them than those of nepheline. The areas of both, 

 when treated with HC1, gelatinize easily and stain with fuch- 

 sine. It must be mentioned that on a first cursory examina- 

 tion the analcite was passed over as nepheline, and it was not 

 until the H 2 determination of the analysis showed a percent- 

 age in striking contrast with the freshness of the rock that the 

 presence of analcite was definitely determined. 



The freshest specimens from near the borders of the dike 

 show characters analogous to the preceding, though they are 

 much finer-grained, and apt to be somewhat decomposed. 



A chemical analysis was made on a specimen from near the 

 center of the dike, obtained by blasting, this being far fresher 

 than those collected by myself. For it, as well as for other 

 material from this region, I am deeply indebted to the liberality 

 of Mr. J. H. Sears. The results are given in No. I. 



The analysis is seen to be that of an almost normal tinguaite; 

 silica, iron oxides, magnesia and lime not varying materially 

 from the figures of other analyses. Soda is, however, higher 

 and potash very much lower than in any other, this rock being 

 the poorest in K 2 of any tinguaite yet fully described. H a O 

 is high for a rock of the freshness of this one. 



