Porphyritic appearance of rocks

Abstract

Object of Paper The variation of the texture of igneous rocks as the margin is approached has long been recognized, and references to marginal and “Rand facies” phenomena are not infrequent. Yet generally they lack exactness, for in but few cases have the exact distances of specimens of an igneous rock from the margin been noted. Yet a little thought would lead one to expect important aid to the petrographer who wants to know the origin of things from studying just these marginal sections, and especially a series at known distances from the margin, since every one knows what a difference has been made in paleontology by the studies of successive stages in the development of a form, and we may find similar light at the margin of an igneous rock on the different generations of crystals and phenocrysts. For instance, if, as Michel-Levy says, there are two generations even . . .