Geology - AHSEC Higher Secondary (Assam)
What the paper tests
Geology in the Assam Higher Secondary (AHSEC) examination is one broad domain, and it rewards a candidate who can name, classify and correctly place a specimen, a fossil or an outcrop. Expect short-answer questions on crystal systems and symmetry, Mohs hardness and cleavage, igneous–sedimentary–metamorphic classification, fossils, strike and dip with fold and fault terminology, plus the stratigraphy and economic deposits of Assam and the North-East.
Core content
- Crystallography: six systems, symmetry elements (plane, axis, centre), and Miller indices for faces.
- Mineralogy: Mohs scale from talc (1) to diamond (10); silicate structures from isolated tetrahedra (olivine) to frameworks (quartz, feldspar); cleavage angle separates amphibole (about 56°/124°) from pyroxene (about 87°).
- Petrology: igneous rocks by silica content and colour index; sedimentary rocks by grain size and cement; metamorphic rocks by facies and index minerals.
- Structure: apparent versus true dip; a fold has a hinge and axial plane; a fault has throw, heave and slip.
- Assam succession, oldest first: Precambrian gneissic basement (Shillong and Mikir hills), Permian Gondwana coal measures, Eocene Jaintia Group (Sylhet Limestone, Kopili Shale), Oligocene Barail sandstone, Miocene Surma, Tipam, Dupitila, then Brahmaputra alluvium.
- Economic geology: Digboi and Naharkatiya oil, Makum coal, Umrangshu limestone, Naga ophiolite.
Worked example
A dark-green specimen has hardness 5.5–6, two cleavages meeting near 87°, vitreous lustre, no useful streak, and comes from a gabbro. Hornblende is ruled out because its cleavages meet at roughly 56° and 124°; olivine has no cleavage at all; colour, hardness and host rock together point to augite, a clinopyroxene.
Traps that cost marks
Never give a single dip figure when the question asks for both direction and amount. Learn the Assam succession in order - Tipam above Barail, Dupitila above Tipam. Do not confuse body colour with streak, and do not offer a long-ranging fossil such as a gastropod as an index fossil.