The migmatites and felspar-porphyroblast rock of Glen Dessarry, Inverness-Shire

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Abstract
Summary: In Glen Dessarry, Inverness-shire, the core of a sharp anticline affecting regionally injected Moine rocks is occupied by a hornblendic inlier petrographically comparable with the Lewisian of North-West Scotland and chiefly composed of a series of felspathic gneisses, with subordinate pyroxene-amphibole rocks originally sediments, later metasomatized, and abundant pink pegmatite. The felspathic gneisses include basic, dioritic and syenitic types, the last occupying the centre of the inlier. They are considered to have resulted from migmatization of pyroxene-amphibole rocks by pegmatitic material and constitute an evolutionary sequence of decreasing basicity. Movements within the gneisses during their formation impressed steeply inclined directional structures upon them, and are believed to be of post-Moine age. With the decline of the stresses that accompanied migmatization the felspar mosaic of some gneisses recrystallized to form larger alkali-felspars of good outline, originating a series of porphyroblastic gneisses of varying basicity. The most striking of these is a rock with the appearance and chemical composition of a syenite, almost wholly composed of randomly orientated, large, grey alkali-felspars, and termed “felspar-porphyroblast rock”. The dominant factors in the formation of this rock are believed to have been stress decline, elevated temperatures and the promoting action of interstitial fluids residual from migmatization.