These letters concern various scientific subjects, such as fishes, insects, snakes, mice, fossils, and mosses. Other subjects include learned institutions, notably the Smithsonian Institution and the Academy of Natural Sciences of Philadelphia, and men of science. Correspondents include S. S. Haldeman, Walter J. Hoffman, Spencer F. Baird, Edward D. Cope, and Leo Lesquereux.

Principally covering botany and agriculture, this collection includes many manuscripts on trees, shrubs, and plants of different species, copies of botanical essays by others, essays on fruit trees, etc., by Auguste Denis Fougeroux de Bondaroy (1732-89), notes and drafts for the latter's revision of Duhamel's Traité des Arbres et Arbustres. Also miscellaneous essays, sketches, and memoranda on bones of birds and animals, electricity, fish, steam engines, ventilation, temperature and air pressure, mathmatics, paleontology ("Observations sur les os d'éléphants fossiles"), chemistry, metallurgy, entomology, architecture, taxidermy ("Méthode pour empailler les oiseaux"); lists of plants; notes on England, Canada, Mexico, China; notes of reading in Pliny, John Evelyn, Alexander Russell, William Derham, and others. An unpublished translation of Jethro Tull's Horse-Hoeing Husbandry, with additions and revisions by Duhamel du Monceau. An alphabetical catalogue of Duhamel's gardens, prepared by Fougeroux de Bondaroy. The collection has manuscript material in pre-publication form along with published material with marginalia for improvements of later editions. There are a multitude of sketches, botanical materials and seed packets. The collection includes lists of American trees and seeds shared with European scientists. Benjamin Franklin acts as a go between French and American botanists and John Bartram sends seeds to France from his garden. Also correspondence (ca. 170 pieces) with, among others, Peter Collinson, Duc d'Aven, Duc de Noailles, Louis J. M. Daubenton, Mathurin Jacques Brisson, Jean François Gauthier, Comte de La Galissonière, Chrétien Guillaume de Lamoignon de Malesherbes, Emerich Vattel. The collection has been described in part by Gilbert Chinard, "Recently Acquired Botanical Documents," APS Proc. 101 (1957): 508: and by Joseph Ewan, "Fougeroux de Bondaroy (1732-1789) and his Projected Revision of Duhamel du Monceau's Traité (1755) on Trees and Shrubs. I. An Analytical Guide to Persons, Gardens, and Works mentioned in the Manuscripts," APS Proc. 103 (1959): 807.

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.