Pg. 51, 52, 53, pl. 7. Series of schists and greenstone in Newton copper mine area (near Jackson, Amador County, California) tentatively correlated with the Jurassic Amador group described by Taliaferro, 1942 (GSA Bull., v. 53, no. 1), is subdivided into 4 formations (descending) Mountain Spring volcanics, Dufrene slate, Newton Mine volcanics, and Sunnybrook volcanics (all new). Dufrene consists of sequence of dark-blue-gray slates and fine-grained gray sandstone; some green chloritic feldspathic schist near base. Approximate thickness 745 feet. In absence of fossil evidence, wider regional studies will be necessary to determine definitely whether Dufrene slate and overlying Mountain Spring volcanics should be considered uppermost Amador group or part of Mariposa slate. Measured section, along Mountain Spring Creek from Mariposa slate eastward, dips steeply to east; relation of beds to Mariposa slate suggests section is overturned, and is on west limb of an overturned anticline or its faulted equivalent. Underlies Mountain Spring volcanics; overlies Newton Mine volcanics. Age is Jurassic, based on tentative assignment to Amador(?) group. Mapped in Newton mine area (scale 1:400).
Named from exposures near Dufrene Ranch, west of Newton mine (6 mi west of Jackson [Sutter Creek 15-min quadrangle] Amador Co., east-central CA). [No type designated] but measured section along Mountain Spring Creek is about 745 feet thick.

Source: US geologic names lexicon (USGS Bull. 1200, p. 1172-1173); GNU records (USGS DDS-6; Menlo GNULEX).