Many letters were written before 1848, apparently antedating Rau's deep involvement in anthropology. Some concern political events in Germany. Two letters written after 1850 are from the artist Heinrich Balduin M llhausen. Still other letters were sent by Rau to Carl Hermann Berendt. These reflect their personal relationship and include references to American politics, Smithsonian administration, and miscellaneous developments among anthropologists.
Most correspondence is in German script.
Rau's catalog of his own archeological collection, now in the National Museum of Natural History, is in the series of Numbered Manuscripts . The Oglala Dakota leader Red Cloud and Marsh, a well-known paleontologist, had known one another since 1874, when Marsh was at the Red Cloud Agency doing field work. The paleontologist heard the Indian's complaints against Indian agent J.J. Saville, and his subsequent appeals on Red Cloud's behalf led to an investigation and removal of Saville. In 1883, Red Cloud visited Marsh at the Peabody Museum of Yale University and was photographed there. The print is a copy from a photograph in the Peabody Museum at Yale University.