This is primarily a collection of letters, with some additional documents, concerning Young's interest in botany, geology, mineralogy, and natural history. There is information about Bowdoin College, where he studied under Parker Cleaveland. There is also much on the natural history of Maine, where he was the State Botanist in 1847-1849, and also on the Bangor Natural History Society. There are materials on Brazil in relation to Young's service there as the U.S. consul to Rio Grande do Sul from 1863 to 1873. There are also letters from his brother, John C. Young, and his sister, Sarah Augusta Young.

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.