KOnlg.] '->18 [Jan. 19, 1877. 



Owing to the extreme cleavage of the mineral, a sufficiently thin section 

 could not be prepared to allow a microscopical and optical examination ; 

 and the presence or absence of minute particles of other species could 

 therefore noi be ascertained. The analysis was conducted with great care, 

 of every constituent a check determination was obtained. Three alkali 

 determinations gave identical results. Only the ferrous iron was but once 

 determined. After being in sealed tube for nearly a week, the mineral 

 was not completel}' decomposed. After titration I filtered from silica and 

 undecomposed part, removed silica by sodium hydrate, and from the 

 weight of residue I calculated the ferrous iron pro toto. In the face of the 

 duplicate and closely agreeing determinations I cannot explain the defi- 

 ciency' of two per centum in the analysis. 



3. Zircon. 



Occurs in brilliant, mostly iron black crystals, imbedded in quartz or in 

 astrophyllite. The size varies from microscopic dimensions to ^ inch py- 

 ramidal edge. 



CrystallofjrajiJiical properties. The form is a combination P. X)P. OP. 

 I observed the basal plane on every crystal which I examined. It is some- 

 times very small, but mostly very plainly developed and splendent. On one 

 crj'stal it appeared distinctly rhomboid, and this induced me to measure 

 the angles of the pyramidal edge, which I found 120° 50' ; 123° 15'. But 

 all other cr^'stals gave as nearly 123'^ as the uneven faces could be expected 

 to average. Tlie rhomboidal crystal stands, therefore, alone as a distorted 

 individual. The prism is generally very small, often only a splendent, edge. 



When boiled with strong hydrochloric acid the black color disappears, 

 and the substance exhibits a yellowish flesh color. The individuals appear 

 corroded. Otherwise, physical characters identical with those of other 

 localities. 



100.18 



The coarse powder of the mineral had been boiled for some time with strong 

 hydrochloric acid, until no more iron was dissolved. The unusually large 

 percentage of iron still shown by the analysis must reasonably be consid- 

 ered as belonging to the mineral. 



Note. — While this paper is in press, I obtained other specimens from 

 El Paso County, in which a black mica occurs in large foliated masses, tra- 

 versed by astrophyllite. This and another mineral, apparentlj' arfvedsonite 

 changed into a micaceous mineral, I am investigating at present. 



