Series I is a miscellaneous collection of letters concerning geology, geological exploration of Russia, entomology, glaciers, appointments in the British Museum, Geological Society of London business, Royal Geographical Society, references to David Livingstone, and zoology. Series II consists of letters written from America, to Murchison, discussing geology, natural history, and contemporary topics.

The collection is a miscellaneous group of letters of mainly British scientists and physicians, purchased as an existing autograph collection. There are a few American signatures. The letters are primarily from the nineteenth century and focus on medical and geological topics, but also there are some earlier and later dates. In addition to the letters are anatomical drawings of surgery, sketches of bones, and one geological notebook.

The natural historian Richard Harlan was a pioneer in the study of comparative anatomy and vertebrate paleontology in the United States during the years following the War of 1812. Having received his medical degree from the University of Pennsylvania in 1818, Harlan was employed as an instructor of anatomy at Joseph Parrish's school and at the Philadelphia Museum. A practicing physician and member of the Academy of Natural Sciences of Philadelphia and the American Philosophical Society, Harlan made important contributions in comparative neuroanatomy, paleontology, herpetology, and systematic zoology. He died shortly after moving to New Orleans in 1839. Harlan's journals document two of the three overseas voyages he undertook during his lifetime. The first took place in 1816-1817 when Harlan was still a medical student, accompanying an East Indiaman to Calcutta as ship's surgeon. The second took place when Harlan was at the peak of his career in 1833, venturing to England, France, and Italy to strengthen contacts with European colleagues. Interesting travel narratives in themselves, the journals mingle enthusiasm for the new and exotic with a touch of Harlan's truculance. The European journal includes a valuable account of the 3nd meeting of the British Association for the Advancement of Science at Cambridge at which Harlan delivered a paper on fossil reptiles.

There is correspondence concerning geology, botany, natural history, natural selection, evolution, British science and scientists, and other scientific topics. Most of the letters are written by Lyell, with a very few items by others.