James Douglas (1753-1819) was a polymath, well ahead of his time in both the fields of archaeology and earth-sciences. His examinations of fossils from the London Clay and other geological formations caused him to conclude that the Earth was much older than the 4004 BC allotted to it by his contemporaries. He had come to this conclusion by 1785 and published these findings in that year, long before other researchers in the same field. His Nenia Britannica, published in 1793, reveals a remarkably accurate grasp of the dating of Anglo- Saxon burials; further illuminated by the contents of his common-place book for 1814-16, discovered by the author in a second-hand bookshop. This common-place book, correspondence with his contemporaries and other sources resulted in the present publication recounting his archaeological and other activities in Sussex during the first two decades of the 19th century.