Abstract
The second meeting of the Washington Mineralogical Society was held as a feld party on April 28,1923. Several mineral localities near Leesburg, Virginia were visited. Ten members, W. S. Burbank, W. F. Foshag; F. L. Hess, E. S. Larsen, C. L. Ross, E. Samson, W. T. Schaller, E. V. Shannon, E. T. Wherry and R. W. G. Wyckhofi, took part. At a quarry in Leesburg a vein of datolite cut the limestone conglomerate of the Newark formation. Crystallized specimens of datolite associated with calcite and barite and an unidentified mineral were obtained together with specimens of xonotlite from the adjoining limestone. At Goose Creek the quarry is cut by a diabase sill. Shear planes carry prehnite, apophyllite, laumontite and calcite. Numerous diabase pegmatites of varying types occur and brought forth much discussion from the members of the society. They are largely plagioclase feldspar with diallage, often in plumose groups several inches long. Miarolitic cavities are abundant in the lighter colored pegmatites and carry albite, byssolite, green pyroxene epidote and titanite. The material collected is now under investigation by Mr. E. V. Shannonof the National Museum.
