�y��vlog

Exceptions to this sequence are unusual; sometimes the first of the felspars have preceded the hornblende or biotite which may envelop them in ophitic manner.

In a very large number of the rocks of this group the plagioclase felspar has crystallized in large measure before the pyroxene, and is enveloped by it in ophitic manner exactly as occurs in the diabases.

When these rocks become fine-grained they pass gradually into ophitic diabase and dolerite; only very rarely does olivine enclose 378 felspar in this way.

In the less complete stages of alteration, ophitic structure may persist, and the original augite of the rock may not have been entirely replaced by 689 hornblende.

Ophitic structure is commonest in olivine-dolerites, though the olivine takes no part in it.