A variety of topics are discussed in these letters, mainly on plant specimens and botanical subjects in relation to England, India, and other parts of the world. The most voluminous correspondence is with Henry Walter Bates, Sir Henry Cole, Sir Richard Strachey, Dawson Turner, and William Crawford Williamson.

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.

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.