The reference is to Salvator Rosa. Ruskin's depreciation of Rosa derives its point here from the fact that Rosa's rocks and mountains were the basis on which Rosa's reputation was built.
A context for this passage on the generalisation of rock forms is to be found in Reynolds Discourse Four of 1777:
perfect form is produced by leaving out particularities, and retaining only general ideas ( Reynolds, Discourses, p. 57)
and
The general idea constitutes real excellence. ( Reynolds, Discourses, p. 58)
It is a view which is found in Vasari, and is associated with the Florentine painters, and with Michelangelo in particular. It was associated with the neo-Platonic thinking deriving from the work of the Platonic Academy in Florence and from the thinking about the nature of art found in Plotinus (particularly Ennead V.8). The Enneads were translated by Ficino in Florence in 1492.