The "Gap Theory"
The Gap Theory, espoused by Dr John Thomas, was published in 1814 by Thomas Chalmers, and in 1910 in the Scofield Reference Bible.
It involves reading Genesis 1:1 as a complete sentence as in the KJV (In the beginning God created the heaven and the earth.) and then assuming that untold ages passed, leaving the remains of their history underneath the waters described in Genesis 1:2 when the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.
A short historical account of the Gap Theory by Prof. Ron Numbers is at The Meaning of Genesis 1 hosted by Counterbalance.org. Extracts:
In 1814 the Scottish divine Thomas Chalmers attempted to harmonize the evidence of vast geological ages with the Genesis story by inserting an indefinite period of time between the initial creation "in the beginning" and the much later creation in the Garden of Eden. This so-call gap theory enjoyed great popularity among Christians eager to harmonize science and religion.
In 1910 the Scofield Reference Bible, an immensely influential annotated edition of the King James Version, presented the gap theory as Christian orthodoxy, influencing millions of Fundamentalists and Pentecostals until late in the century.
Following the lead of John Thomas, Robert Roberts taught the Gap Theory and pre-Adamic creation in The visible hand of God (1883)
Bro James McCann mentions the Gap Theory here:
However, many of the fossils and evidences of past life found by scientists could be the remains of ages and worlds before our own, that preceded the Genesis creation, of which the Bible says nothing. Some have tried to hypothesise where the Angels came from, but this enters a realm unrevealed in scripture.
The Gap Theory is countenanced, and understood to be common among Christadelphians, by Bro L G Sargent here (...any beings of a creation prior to Adam indicated by archaeological remains...) and here (What happened to any pre-Adamic race, we cannot know; that there may have been such beings has never been denied...). Similarly Bro H. P. Mansfield here (If there was a pre-Adamic creation, it must have been overwhelmed by some terrible catastrophe ...).
Christadelphian writers who have argued explicitly against the Gap Theory include Alan Hayward in God’s Truth (1973), Brian Hurn, The Genesis Creation (c. 1985) and Alan Fowler, A drama of creation (1996).
C. C. Walker wrote in favour of the Gap in the first decade of the 20th Century, but against it in The Word of God (1926).
“. . . there does not appear to be any evidence at all that some six thousand years ago an existing cosmos was reduced to such a chaos as is described in Genesis 1v2”
— cited at T4C: The Archaeological Evidence#Neolithic
