MINERALS OF SQTAKK lU'TTK DARK ROCK. 413 



dissolve sliixlitly in HCl, give a small amount of gelatinous silica, with 

 reactions tor Na and none for CI, 11./) or CO,. It must ])e present in the 

 rock only as a rare accessory mineral, and tlie recognition in tlie thin- 

 sections of an occasional patcli is rendered dillicult ])y tlie jractically 

 uniaxial character of some of the orthoclase. 



Cancrinite. — This is indicated hy tlie fact that the rock j^owder ohtained 

 at a specitic gravity of 2.47 dissolved in IICl with gelatinization, and in 

 dissi>lving slowly and continuously gave off CO.,, while carhonates, which 

 wouKl have been thrown down at a higher sj^ecitic gravity, are absent in 

 the rock, as seen in thin-sections. It can l)e i)resent only in very small 

 amount, and the certainty of recognizing an occasional piece in the sec- 

 tion is diininished by the common occurrence of natrolite. The two 

 minerals are alike in their appearance in fibers with parallel extinction. 

 The cancrinite has, it is true, a higher double refraction, but sections may 

 be as low as natrolite, and only by establishing the uniaxial character 

 can the cancrinite be definitely determined. This we have not been able 

 to do, and its presence is therefore only inferential. 



Sodalite. — This also occurs as an accessory component. The rock 

 powder separated below a specific gravit}^ of 2.40 consists partly of this 

 mineral, together with some zeolites. It dissolves readily in HCl anc. 

 HXO3, the solution in the latter yielding a precipitate with AgNOa ^^^^ 

 none with BaCL, tlius showing the presence of sodalite and absence of 

 hauyn or nosean. In thin-section it is very clear and limpid, but con- 

 tains little interpositions somewhat like the feldspars. The actual amount 

 of sodalite in the rock is very small, and this is shown also by the small 

 amount of chlorine obtained in the analysis, part of which belongs to the 

 ai)atite j^resent. 



Natrolite. — The presence of zeolites is indicated by the water obtained 

 in the analysis. Some analcite may occur, but the chief zeolite is natro- 

 lite. which is present in consideral)le amount. It is recognized l)y its 

 parallel extinction and positive character, l)y the small angk; of the 

 optic axes, and by the strength of its doul)le refraction, which compared 

 with the feldspars, rises to .01(J-.()12. It occurs in characteristic bundles 

 of libers, and is in ])art secondary after sodalite and in part after albite 

 and anorthochuse. The (ibcrs are plainly i^vaw eating their way into tlie 

 felds[)ar, and in a given crystal they do this according to a detinite 

 oriented <lirection, as the different pat<*h(!S in the|crystal always have the 

 same orientation. 



Chemical Comi)f).siunii. — The clieinieal compDsiliou of the rock is 

 shown in tlie following anal^^sis. In it the minute trace of CO.. due to a 

 little possible cancrinite is not deteriiiiiie<l, nor is the amount of rarer 



LIX-Riri.i.. Gr.oi.. Sor. Am.. Vol. r,, ImoI. 



