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These basic holocrystalline rocks form a large and numerous class which can be subdivided into many groups according to their mineral composition; if we take it that typical gabbro consists of plagioclase and augites or diallage, norite of plagioclase and hypersthene, and troctolite of plagioclase and olivine, we must add to these olivine-gabbro and olivine-norite in which that mineral occurs in addition to those enumerated above.

That is, the metamorphic rocks are characteristically holocrystalline, while igneous rocks are porphyritic, or cryptocrystalline.

In the Cromdale Hills in the south-east of the county the metamorphic series comprises schistose quartzite, quartz-schists, micaceous flagstones and mica-schists, which are granulitic and holocrystalline, the dark laminae in some cases containing heavy residues such as ilmenite and zircon.

It seems therefore that a holocrystalline pyroxene-garnet rock may be one source of the diamond found in blue ground.

All these types are holocrystalline, and range in texture from coarse granite with augen an inch long down to a fine epidote granite with scarcely visible crystals.