"This index is the third supplement to Special Report 52, Index to the Geologic Maps of California to December 31, 1956, by R. G. Strand, J. B. Koenig, and C. W. Jennings, published in 1958. The first supplement was Special Report 52-A, Index to Geologic Maps of California, 1957-1960, by J.B. Koenig, published in 1962, and the second supplement was Special Report 52-B, Index to Geologic Maps of California, 1964-1964, by J. B. Koenig and E.W. Kiessling, published in 1969. The publication of this map index series is in response to public need for handy, up-to-date indexes to geologic maps of areas in California. These map indexes are also an essential source of information for geologists working out of the Division's district offices."--Introduction (page 5).