Felsic to mafic volcanic rocks and minor interbedded argillite and volcaniclastic conglomerate. Includes basaltic to andesitic pillow flows, porphyritic andesite, andesitic to rhyolitic breccia and tuff, and agglomerate (Eberlein and others, 1983; Brew and others, 1984; Gehrels, 1992; Brew, 1996). Unit is locally metamorphosed to subgreenschist or lower greenschist facies (Gehrels, 1992). Ordovician plutons (predominately Middle Ordovician) of unit Ogi are coeval and were probably the magma source of the volcanic rocks (Gehrels and Saleeby, 1987). On Prince of Wales Island, includes pale-colored dacitic and rhyolitic breccia, tuff-breccia, tuff, and extrusive domes (Brew, 1996) and andesitic and basaltic flows and breccia (Herreid and others, 1978; Rubin and Saleeby, 1992). Age constrained by stratigraphic relations with sedimentary rocks of the Descon Formation (included in unit SOd), intrusive relations with plutons of Late and Middle Ordovician age (unit Ogi), and K/Ar ages of volcanic rocks that indicate early Silurian and Late Ordovician emplacement (Eberlein and others, 1983; Gehrels and Saleeby, 1987)