Series I: General correspondence, 1814-1903

The general correspondence, previously housed at the Peabody Museum, is predominantly scientific in nature. There are more than twelve thousand items, beginning with letters written to and from Marsh following his return from Germany in 1865 and continuing until his death in 1899. Among the socially and scientifically prominent correspondents represented are Cleveland Abbe; Alexander and Louis Agassiz; Spencer Baird; Phineas Barnum; Albert Bierstadt; George Jarvis Brush; William Fredrick Cody; James Dwight Dana; Charles Darwin; Ulysses S. Grant; Asa Gray; Joseph Henry; Ferdinand Hayden; Leonard and Thomas Huxley; Simon Newcomb; Benjamin Silliman, Sr. and Jr.; Herbert Spencer; and Eli Whitney. Relatively few of Marsh's own letters have survived, except for occasional rough drafts that he retained. After the advent of the typewriter, however, his side of the correspondence is represented more frequently.

The most voluminous correspondence is with his co-workers and "bonediggers" such as C. E. and M. P. Felch, George Bird Grinnell, John B. Hatcher, Arthur Lakes, Benjamin F. Mudge, Samuel W. Williston, and W. H. Reed. When each specimen was received in New Haven, it was assigned an identifying number, which was also inscribed within a parallelogram on the accompanying letter or letters.

Marsh's fossil collecting reached its peak in the 1870s and 1880s. His autocratic disposition led to difficulties with some of his colleagues. His lifelong rivalry with Edward C. Cope, who was his junior by nine years and who also aspired to the presidency of the National Academy of Science but was never elected, was a discredit to both men. Their quarrel is sparsely documented in the correspondence, except for an exchange of letters in 1873. There is also an excerpt from a letter written in December of the same year by one of Marsh's loyal diggers, announcing that Cope had inspected Marsh territory and that Davendorf "... discovered marks distinclty made, I presume, by Cope, so I judged he could strike certain places next spring. These marks are now defaced by the sole of my boot, but I know where they are."