The date range is established by the headed notepaper, which is of a sort that CD used from November 1874; he died in April 1882.

The enclosure has not been found. The image included with this letter in the Wellcome Library is an wood-engraving from 1883 based on a photograph by Leonard Darwin.

The so-called ‘parallel roads’ of Glen Roy are three terraces that run parallel to one another along the sides of Glen Roy in Lochaber, Scotland. In the nineteenth century several attempts were made to account for their formation; in a paper published in 1839 in the Philosophical Transactions of the Royal Society of London (‘Parallel roads of Glen Roy’), CD argued that the roads were the remains of beaches formed by the sea as the landmass of Scotland rose in graduated steps. By the time of this letter, he had been convinced that during a great Ice Age ice-flows trapped a series of lakes in the glen and that the roads represented the shorelines of three of these former lakes. See Correspondence vols. 2, 9, and 10 and Rudwick 1974.