Pg. 7. Blacktail formation. Is here defined as including all rocks between Burke formation below and Wallace formation above. Represents undifferentiated equivalents of Revett and St. Regis formations of Coeur d'Alene district. Lower part is prevailingly quartzite; upper part is mostly argillite. Whole formation is distinguished from all others in district by a pinkish or reddish purple color. Lower 5,000 feet which is mostly quartzite, may have only a faint pink cast, but even in these beds thin partings of red shale are common. The lowest beds closely resemble Burke formation, but faintly pink color serves to distinguish them. The upper Blacktail beds are alternating red and green arglllites, with red predominating. In lower part of formation the red argillite occurs only as partings. Argillite increases upward in formation and in upper 3,300 feet it greatly predominates. Uppermost beds, near transitional boundary with Wallace formation, are green, but those mapped with the Blacktail have a peculiar waxy appearance. Age is pre-Cambrian (Belt).
Type locality: Blacktail Mountain, near Talache, Bonner Co., Pend Oreille district, northern ID. The southwest shoulder shows the lower beds and the northerly spurs
show the upper argillitic portion.

Source: US geologic names lexicon (USGS Bull. 896, p. 208).