John Smith Flett

“ Many dolerites are porphyritic and carry phenocrysts of olivine, augite and plagioclase felspar (or of one or more of these) . Others, probably the majority, are non-porphyritic, and these are generally coarser grained than the ground-mass of the former group, though lacking their large conspicuous phenocrysts. The commonest type of structure in dolerite is the ophitic, which results from the felspar of the rock having crystallized before the augite; the latter mineral forms shapeless masses in which the idiomorphic felspars lie. ”

“ Some of the porphyritic dolerites have ophitic ground-masses; in others this structure is imperfect (subophitic) ; while in many the augite, like the felspar, occurs as small and distinct individuals, which react differently on polarized light, and have the outlines of more or less perfectly shaped crystals. Ophitic structure is commonest in olivine-dolerites, though the olivine takes no part in it.
The quartz-dolerites are an important group, hardly less common than the olivine-dolerites. ”

“ They are much employed for road-mending and for kerbstones, though their dark colour and the tendency they have to weather with a dingy brown crust make them unsuitable for the better classes of architectural work. ”