rock cake (sometimes rock bun, as in some late 19th-century references cited by the OED), a fairly plain and solid small cake/bun, usually enlivened with raisins (or currants) and candied peel. Rock cakes take their name from their irregular craggy appearance, not from their consistency. They are made by baking uneven lumps of dryish dough on baking sheets.
Recipes dating back to the 1860s are recorded; a range of flavourings including mace, lemon zest, and brandy were used in early examples. Arnold Palmer (1952) has a reference to City gentlemen in London in the 1870s standing at a counter and lunching ‘off a glass of sherry with a rock cake or a couple of biscuits’. The use of currants, which became standard in the 20th century, is attested from the 1880s.