Hadean
The Hadean () is a geologic eon of the Earth pre-dating the Archean. It began with the formation of the Earth about 4.6 billion years ago and ended, as defined by the ICS, 4 billion years ago. , the ICS describes its status as ""informal"". Geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks on Earth. W. Brian Harland later coined an almost synonymous term, the "Priscoan period", from "priscus", the Latin word for "ancient". Other, older texts simply refer to the eon as the Pre-Archean.
Ancient history
The period between AD 320–550 is known as the Classical Age, when most of North India was reunited under the Gupta Empire (c. AD 320–550). This was a period of relative peace, law and order, and extensive achievements in religion, education, mathematics, arts, Sanskrit literature and drama. Grammar, composition, logic, metaphysics, mathematics, medicine, and astronomy became increasingly specialized and reached an advanced level. The Gupta Empire was weakened and ultimately ruined by the raids of Hunas (a branch of the Hephthalites emanating from Central Asia). Under Harsha (r. 606–47), North India was reunited briefly.