What we know
Rhyolite quarry on a D&RG spur south of Castle Rock; Scott also marks O'Brien Quarry (spur 1880-1905) and Hathaway nearby.
Where it was
'Madge Quarry (X)': Scott, Historic Trail Map of the Denver 1x2 Quadrangle (USGS I-2639, 1999, S-0106), symbol read from the reprojected sheet (good to a few hundred feet). Place Names of Colorado: Sec. 23, T8S R67W, 'Est. by 1916. RR stop on D&RGW. 5 mi S of Castle Rock.'; this point is 0.29 mile from that section's center.
Read from Glenn R. Scott's USGS historic trail map of 1999 and confirmed by a second, independent source.
Sources
Sources are named in the text above. Numbers such as S-0080 refer to our source list. Most newspaper quotations were read in the Colorado Historic Newspapers Collection. Long quotations from recent books are shortened here.