Volume9, page 154, 1924

    AN OCCURRENCE OF WITHERITE IN THE ALTYNLIMESTONE AT MANY GLACIER, MONTANA

    MARGARET BRADLEY FULLER, Northwestern University

    During the field season of 1923 considerable witherite was observed filling cavities in the Altyn limestone of the Belt series in Glacier National Park. The chief exposures appear in the lower beds of the Altyn limestone immediately above the plane of the Lewis overthrust fault, along the gorge below Swiftcurrent Falls at Many Glacier. There are eighty to one hundred feet of the jointed platy limestone full of cavities containing the witherite.

    The Altyn limestone in this locality is a highly siliceous calcium-magnesium-carbonate rock containing rounded to angular quartz and feldspar grains. The fresh surfaces are bluish-gray and very dense. The weathered surfaces are light buff to brown and dotted with etched grains of sand.

    The witherite appears: (1) in flat masses one to six inches in thickness parallel to the bedding; (2) in thin lenses one to three inches thick and six to eighteen inches across; and (3) in very irregular lumps up to two feet in diameter. The mineral fills the cavities in pure translucent masses which are not associated with other minerals. The analysis shows 98.1 % BaCO3, 0.14% of SiO2, and about one and a half percent of calcium and magnesium carbonates. The masses are colorless to pale buff and interlock in brush and fan shaped growths which extend from the walls of the openings towards the centers so as to nearly fill them. The contact with the limestone wall rock is very distinct. In many cases where the cavity fillings have been entirely removed the surface at the limestone contact is pitted with the impressions of the sand grains which lined the openings before the witherite was deposited. In no case were the sand grains observed included within the witherite. Evidently the openings were completed before the witherite began to fill them; replacement was not simultaneous with the solution of the wall rock.