312 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



CUSTER COUNTY 



The complex sodium-calcium silicates of the scapolite group occur 

 rarely in the contact-metamorphic silicate rocks of the Aider Creek 

 (Mackay) district. 35 



LEMHI COUNTY 



A specimen of altered calcareous schist from the northeast corner 

 of the Columhia mill, in the Yellow Jacket district, Lemhi County, 

 contains many little veinlets of calcite, scattered quartz grains, pale 

 green hornblende, poikilitic mizzonite, and a few flakes of biotite, 

 each intergrown with or included in the other. 36 



SHOSHONE COUNTY 



In the St. Joe- Clearwater region in the southern portion of Shoshone 

 County, and mainly included in the area of the Avery quadrangle, 

 scapolite is abundantly developed by metamorphism in the Newland 

 member of the Belt sedimentary series. The effect of metamorphism 

 is clearly manifested in nearly the whole of the area by the argillaceous 

 layers of this formation, which become altered, for the most part, to 

 chocolate-colored biotitic hornstones of fine texture in which roundish 

 grains of scapolite whose average diameter is about that of buckshot 

 are abundant. On weathered surfaces the scapolite grains are white, 

 in strong contrast with their dark matrix, from which they project 

 in relief. 37 



Through the courtesy of F. C. Calkins a large number of specimens 

 of these scapolite-bearing rocks have been available for examination. 

 The specimens containing rounded or nearly spherical bodies are most 

 abundant. Typical ones of these are shown in Plate 7. The masses 

 of scapolite vary in size and become larger and very poikilitic until 

 they coalesce and form masses which on weathered surface have an 

 organic aspect and resemble fossil algae. 



One specimen examined from the St. Joe River, about 1 mile 

 above Siwash Creek, contains nearly spherical and shot-like masses 

 of scapolite which make up over half of the rock. These average 

 2 mm. in diameter and are prominent on weathered surfaces. They 

 break free from their matrix, which is fine scaly sparkling dark brown 

 material. The scapolite bodies are whitish and opaque on their 

 surface, but where broken they appear translucent with rather 

 resinous luster. Under the microscope the material of the scapolite 

 masses is extremely poikilitic, inclosing so many grains of other 

 minerals as to make its optical properties difficult of determination. 



3' James F. Kemp and C. Q. Gunther. Trans. Amer. Inst. Min. Eng., vol. 38, p. 321, 1908. 



m J. B. Umpleby. U. S. Geol. Surv., Bull. 528, p. 167, 1913. 



3< F. C. Calkins and E. L. Jones, jr., U. S. Geol. Surv., Bull. 530, p. 79, 1913. 



