Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . stal being formed of zones of different. Fig. 21. — Sphrerulites in felsite. Ground-mass shows aggregate nicols. chemical composition (the successive zones in the plagioclasesgrowing more acid towards the exterior), or by ultra-microscopictwinning,* Fig. 20. ^?, Petrology for Students, 1895, p. 14. 38 IXVESTIGATION OF CHARACTERS OF MINERALS. (r) Aggregate structure, being a confused mass
Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . stal being formed of zones of different. Fig. 21. — Sphrerulites in felsite. Ground-mass shows aggregate nicols. chemical composition (the successive zones in the plagioclasesgrowing more acid towards the exterior), or by ultra-microscopictwinning,* Fig. 20. ^?, Petrology for Students, 1895, p. 14. 38 IXVESTIGATION OF CHARACTERS OF MINERALS. (r) Aggregate structure, being a confused mass of separate littlecrystals, scales or grains all extinguishing at different times, Fig. 21. {d) Sphcerulitic structure, produced by the aggregation, in aradiate form, of crystals or ciystallites. It is generally easily per-ceiv^ed by the dark cross, resulting from the extinguishing of thelight in those crystals whose directions of vibration are parallel tothe planes of vibration of the nicols. When the stage is revolvedthe arms of the cross do not rotate, Fig. 21. {c) PscudojHorphic structure, which may be partial or completeand is noticed by the changed portions producing different optical
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