Abstract

Segregation of early crystals into clusters exhibiting glomero-porphyrit-ic or synneusis structure characterizes many igneous rocks and is especially striking in porphyries. These widespread, but largely neglected, structures are of general petrologic interest for several reasons. First, their formation implies a drifting together and attachment of crystals freely suspended in a fluid medium (synneusis: Vogt, 1921). Such structures, accordingly, provide clear petrographie evidence of igneous origin. Moreover, this process of attachment controls several basic elements of the magmatic fabric, notably: (1) the mutual orientation of adjacent crystals in synneusis relation; (2) the nature of their common boundary; and, (3) the small-scale segregation of minerals within the fabric as a whole. A more general survey of the problem of synneusis will be given in a subsequent paper. The present note is concerned only with the last of these textural features.

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