VIII.—The Petrology of Picritic Rocks in Minor Intrusions—a Picrite Sill on the Island of Soay (Hebrides)

Abstract

A Small picrite sill of critical significance is examined in detail, from top to bottom, with particular reference to the distribution and crystallization of the olivine. Statistical evaluation of the sizes of olivine crystals establishes a gradation from large phenocrysts to small groundmass crystals. The micrometric data indicate that, while the feldspar content has remained constant, there is an inverse relationship between the distribution of olivine and pyroxene. This relationship, viewed in conjunction with the common tendency toward skeletal crystallization of the olivine and its gradation in size, amounts to substantial evidence that much of the olivine rapidly crystallized at the time when the sill was emplaced. At a few places, the sandstones and grits, into which the sill has been intruded, have been fused. Where it has penetrated a dolerite dyke along its direction of strike, a non-porphyritic facies, with very much less olivine, is associated with xenoliths of peridotite and of fused sandstone. It is inferred, from the field relationships, that this non-porphyritic facies was intruded as a liquid, containing xenoliths, in advance of the picrite. The chemical composition of this liquid is more calcic than basalt. It is concluded that this earlier calcic liquid was followed by a later magnesian (picritic) liquid and that these liquids have probably a deep-seated genetic relationship with feldspathic peridotite. Further research must be directed toward investigating this relationship both experimentally and in picrite—peridotite rock associations.