The Historic American Engineering Record (HAER) was established in 1969 by the National Park Service, the American Society of Civil Engineers and the Library of Congress to document historic sites and structures related to engineering and industry. Between 1987 and 1991, the history and archaeology of Salt Spring Valley was studied in advance of mining operations by Meridian Minerals, Inc. Among other products, three HAER recordations of the District'smines and mills were filed with the National Park Service in Washington, D.C.  Copies are also on file at the Calaverasl County Historical Society in San Andreas. 

These reports contain detailed technical histories of mining and milling operations, measured line drawings of mill and mine sites, and historic and modern photographs.  They can be accessed at the HAER website (HAER Madam Felix-Hodson District).  Included are

• HAER No. Ca-76 - Madam Felix-Hodson District gold Mnes and Mills (Fuller and Costello 1990)
• HAER No. CA-77 - Mountain King Gold Mine and Mill (Fuller and Costello 1990); and
• HAER No. CA-81 - Royal Consolidated Gold Mine and Mills (Fuller and Costello 1990).