The month and year are established by the postmark. CD wrote March in error.

No book on laccolites has been found in the Darwin Libraries at CUL and Down, but CD probably refers to a report made by Grove Karl Gilbert on the geology of the Henry Mountains, Utah (G. K. Gilbert 1877). Gilbert devoted a chapter of this work to the analysis of formations he termed laccolites (now more usually referred to as laccoliths), which were masses of igneous rock intruded between rock strata causing uplift in the shape of a dome (ibid., pp. 51–98). The following sections of the book focused on various types of erosion and land sculpture (ibid., pp. 99–150).

A general election was held from 31 March to 27 April 1880 in which the Liberal party won by one of their largest majorities. Both Southampton parliamentary seats were won by Liberal candidates (Craig ed. 1989, p. 280).

Sara Darwin and William visited Down from 25 March until early April 1880; Emma’s diary entry for 27 March reads, ‘poorly as colds for a week’ (Emma Darwin’s diary (DAR 242); letter from Emma Darwin to H. E. Litchfield, 4 April 1880 (DAR 219.9: 229)).