Mikir Hills regarded as the north-eastern corner of the Indian Shield separated by the Rangpur Saddle. In the northeast of the shillong plateau, the Mikir Hills constitute an Archaean inlier, projecting through the alluvium and fringed with more or less horizontal Tertiary beds (66 to 2 million years ago), and in one small area overlain by a probable Mesozoic trap (245 to 66 million years ago). The Mikir Hills are comprised of coarse-grained porphyritic granites and fine-grained strongly banded gneisses. The general strike of the foliation of the gneisses is E-W; the dip is either vertical or steep. The gneisses, which are usually fresh and uncrushed, consist of quartz, plagioclase, rare orthoclase, brown biotite, and green hornblende. Graphic and micropegmatitic structures are frequently seen amongst the quartz and the plagioclase. Occasional veins of quartz are seen in the gneisses. Hypersthene bearing gneisses are also observed at certain places, similar in all respects to the charnockite of Madras, and intimately interbanded along the foliation planes of the gneisses. The thickness of the hypersthene bearing rocks varies from six metres to nine metres. They are coarse-grained and fresh, with little signs of crushing. The groundmass is chiefly composed of quartz grains and slightly decomposed plagioclase with much hypersthene scattered through it. Trap dykes are unexpectedly rare in the Mikir Hills. [Eunuse Akon]
See also shillong plateau.