These are letters written from America, to Murchison, discussing geology, natural history, and contemporary topics.

These are primarily letters, with some receipts for medical services rendered included. They relate to the general social and intellectural life of Philadelphia and to publications on medical topics. Many of the letters are introductions to Europeans when Pennock first visited there. Letters dated post-1867 are to relatives and colleagues (?).

The Scientists Collection is comprised of individual letters and small groups of correspondence from American, British, French, and German scientists during the past three centuries. Although the content is highly varied, there is significant strength in astronomy, natural history, conchology, and geology. Among the scientists better represented in the collection are the astronomers William Radcliffe Birt, J.F.W. Herschel, and Franz Xaver von Zach; the conchologists A.D. Brown, Fred L. Button, Otto Mörch, Alfred Newton, Christian M. Poulsen, Temple Prime, Benjamin Lewis Vulliamy, and A. G. Wetherby; the physical scientists George Biddell Airy, Arnold Sommerfeld, Ferdinand R. Hassler, and Max Planck; the archaeologist Jean François Nadaillac; the philosopher William Whewell; and the naturalists Walter Henry Bates, Robert Chambers, Edme Dupuget, Robert Kaye Greville, Joseph Henry, John Stevens Henslow, John Lubbock, and Herbert Spencer.

John James Audubon (1785-1851), the American Woodsman, is a legendary naturalist and bird artist. His technique of painting North American birds dramatically as they appeared in their natural habitat was a major contribution to the emerging discipline of ornithology in the nineteenth century. His masterpiece, The Birds of America (1827-1838), elephant folio, was followed by a companion text edition, Ornithological Biography (1831-1849), a smaller octavo edition of Birds (1840-1844) and The Viviparous Quadrupeds of North America, published posthumously. This collection of original letters of publication information, ornithology, and some personal notes was sent primarily to Lucy Bakewell Audubon, his wife, from 1826-1834, and to Victor Gifford Audubon, his son, from 1833-1834, 1840-1844, with some sporadic contact with both between 1836-1839. Items in the collection relate to Audubon's Florida, Great Egg Harbor, and Great Pine Forest expeditions but not to his final expedition up the Missouri River. Of particular note, letters of 1833 and 1834 contain references to his response to the Academy of Natural Sciences of Philadelphia president George Ord's attacks on his credibility. A partial journal entry from New Orleans in 1821 and a few letters to other correspondents, including John Bachman, round out the material.

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.