Shillong Massif a massive blow of rock within an orogenic belt, generally more rigid than the surrounding rocks, and commonly composed of crystalline basement or younger plutons are known as massif. Rigid rock masses of the shillong plateau are known as the Shillong Massif. The larger portion of the Plateau comprises of Archaean gneisses intruded by coarse pink granites. In the north the gneisses and the granites are uncovered, but southwards they are concealed beneath Cretaceous (144 to 66 million years ago) and Tertiary deposits (66 to 2 million years ago) and a Mesozoic trap (245 to 66 million years ago). Light grey coloured older gneisses are usually fine grained and distinctly banded. They are mainly composed of feldspar, quartz and biotite. Mylliem granites consisting of large, porphyritic, flesh coloured microcline with subordinate acid plagioclase and orthoclase, quartz, biotite and hornblende; which are exposed over an area of nearly 50 sq km.
The Shillong Series, occupying a moderately large area in south and east of Shillong, comprise of bedded quartzites, highly crumpled foliated mica schists, hornblende schists, granulites, amphibolites and banded hematite quartzite. In the Khasi Hills the strike of the beds is E-W; with a general southerly dip along the northern portion and a general northerly dip among the southern exposures. The Shillong Series is comparable with the Bijawar Series of Bundelkhand and the iron ore series of Chota Nagpur. Other Archaean rocks of the Shillong Massif are hybrid gneisses, epidiorites and sillimanite bearing rocks. [Eunuse Akon]