Gap creationism (also known as ruin-restoration creationism, restoration creationism, or "The Gap Theory") is a form of old Earth creationism that posits that the six-yom creation period, as described in the Book of Genesis, involved six literal 24-hour days (light being "day" and dark "night" as God specified), but that there was a gap of time between two distinct creations in the first and the second verses of Genesis, which the theory states explains many scientific observations, including the age of the Earth. It differs from day-age creationism, which posits that the 'days' of creation were much longer periods (of thousands or millions of years), and from young Earth creationism, which although it agrees concerning the six literal 24-hour days of creation, does not posit any gap of time.
Gap creationism became increasingly attractive near the end of the 18th century and first half of the 19th century, because the newly established science of geology had determined that the Earth was far older than common interpretations of Genesis and the Bible-based flood geology would allow. Gap creation allowed religious geologists (who composed the majority of the geological community at the time) to reconcile their faith in the Bible with the new authority of science. According to the doctrine of natural theology, science was in this period considered a second revelation, God's word in nature as well as in Scripture, so the two could not contradict each other.
From 1814, gap creationism was popularized by Thomas Chalmers, who attributed the concept to the 17th century Dutch Arminian theologian Simon Episcopius. Chalmers became a divinity professor at the University of Edinburgh, founder of the Free Church of Scotland, and author of one of the Bridgewater Treatises. Other early proponents included Oxford University geology professor and fellow Bridgewater author William Buckland, Sharon Turner and Edward Hitchcock.