310 Spun' — Scapolite Rocks from Alaska. 



Art. XXXI. — Scapolite Rocks from Alaska;* by J\ E. Spurr. 



The following rocks containing scapolite as an essential con- 

 stituent were collected by the writer in the summer of 1898, 

 while making a geological reconnoissance in southwestern 

 Alaska. 



Andesine-oligoclase-scapollte-biotite rock. 



This rock was found on the Yentna River (a branch of the 

 Sushitna) some 12 or 15 miles above the junction with the 

 Sushitna, where it forms large masses of uniform appearance, 

 which are probably great dikes cutting an older rock. Speci- 

 mens of this older rock, taken in different places, proved to be 

 hornblende syenite and hornblende diorite. The scapolite 

 rock is coarse-grained and granitoid in appearance, the bluish- 

 gray of the scapolite suggesting quartz at first glance. Under 

 the microscope the structure is hypidiomorphic granular, grad- 

 ing to panidiomorphic. The essential minerals are feldspar, 

 scapolite, and biotite, abundant in the order of naming. The 

 feldspars are mostly twinned according to the albite law, and 

 one crystal examined by the Fouque method showed on a sec- 

 tion perpendicular to the negative bisectrix an angle of 70° 

 between, the plane of the optic axes and the albite twinning, 

 proving it to be an andesine-oligoclase. The scapolite is both 

 idiomorphic and hypidiomorphic, like the feldspar. Some sec- 

 tions are always dark, and give the dark cross of uniaxial 

 minerals in convergent light. The cleavages, which are often 

 broken, intersect at right angles and belong to the faces 100 

 and 010 ; another less perfect cleavage evidently belongs to 

 the prism 110. The relief of the mineral is small, like quartz, 

 while the double refraction is higher, giving a light-yellow 

 color; it is optically negative. The biotite is fresh, and like 

 the other minerals is idiomorphic or hypidiomorphic in habit; 

 it contains grains of apatite. Small crystals of zircon, showing 

 the prism and pyramid faces, are occasionally found. 



Analysis of andesine-oligoclase-scapolite-biotite rock. 

 [Analyst, Dr. H. N. Stokes.] 



Per cent. Per cent. 



SiO„ 62-78 BaO "04 



TiO" 9 .- -56 SrO trace 



CO„_. ..-- CaO 4-84 



P 2 5 -15 MgO 2-32 



S -02 KJO 2-15 



CI trace Na 2 0._ ._. 4-11 



A1 2 3 1M6 Li a O trace 



Fe 2 3 1-96 H 2 at 100° '24 



FeO 2-31 H 2 "OatllO° "88 



MnO -06 



Total 99-5S 



* Published by permission of the Director of the IT. S. Geological Survey. 



