Abstract Two suites of olivine-rich ultramafic and feldspathic rocks appear to be=present in the , Mid-Atlantic Ridge: one which seems to have alkalic affinities, and one similar to the chromitites bearing alpine peridotite-gabbro complexes. The similarities of rocks in the two environments— continental strta oceanic—lmply that much about the petrology of mid-oceanic ridges may be learned from studies.of continental. complexes, and that silicic rocks have been formed in the -mantle. Although gabbros in St. Paul Rocks and similar rocks at Tinaquillo, Venezuela and lizard, England have been interpreted as not comagmatic with intimately associated peridotite by some petrologist, evidence to the contrary at Lizard is discussed. Association of fresh gneissic gabbro, some contaiging quartz, with talcose serpentinite, amphibole schist, quartz diorite and epidotic but unsheared basal to along.the Mid-Atlantic Ridge is believed to indicate presence of alpine-type rocks that occur normally in eugeosynclinal belts. Gabbro, described as partly interlayered with peridotite by gravitational differentiation, forms major parts of three widely separated ultramafic complexes which have been interpreted as slices of oceanic crust and upper mantle: the Troodos massif in Cyprus, the Bowutu Mountains in Papua, and the Camaguey complex in central Cuba. If, as Dietz has suggested, peridotite and related rocks in eugeosynclines represent fragments of ocean rind formed along mid-oceanic ridges and moved laterally by ocean-floor spreading, gabbro must be an essential constituent of the uppet? mantle. This could account for many geophysical anomalies, but would complicate some postulated mechanisms involved in ocean-floor spreading.

Abstract Relict cumulate features preserved in podiform chromite deposits include some textures found in stratiform complexes and, in addition, nodular and orbicular textures. Nodules of chromite are shown to have grown while freely suspended in magma, either by crystallization of chromite alone, or of chromite plus plagioclase and/or olivine in varying proportions. Some chromite nodules have the external form of crystals, but internally all are aggregates of grains or crystals in random orientation. Regardless of form, the nodules constitute packed structures like piles of marbles. In most podiform deposits the relict textures have been modified or destroyed by flowage at magmatic temperatures; extreme deformation produces gneissic silicate rocks and schlieren-banded disseminated chromitites. The podiform chromitites are believed to have formed as layers in supercomplexes analogous to those known to have produced stratiform type chromitite, by gravitational differentiation of fluid magma in the upper part of the mantle. The chromite of podiform deposits is characterized by high MgO: FeO ratio and reciprocal variation in Cr 2 O 3 and A1 2 O 3 ; primary variation in total Fe content is much smaller than in the stratiform deposits. The layered differentiates appear to have been re-emplaced into the crust as hot crystal mushes. During re-emplacement chromite was solid and relatively rigid compared to olivine, pyroxene, and plagioclase which, although also solid, yielded more or less plastically by crushing and recrystallization. However, some interstitial magma able to form dilation dikes was present. Random distribution of gabbro and peridotite in many alpine complexes and erratic compositional variation of chromite in neighboring deposits indicate mixing of the various differentiates during re-emplacement. A few masses of feldspathic chromitite found in feldspar-free peridotite are attributed to sinking of broken-up dense layers of chromitite (sp. gr. 3.8–4.2) from troctolitic into peridotitic crystal mushes (sp. gr. 2.8–3.2) during intrusion. To produce the thicknesses of massive chromitite and volumes of dunite and olivine-rich peridotite in alpine complexes requires very large volumes of magma, or primary magma more mafic than tholeiite, and differentiation trends very different from those of known stratiform complexes.