The development of the geological concept of “deep time” was based on the principles of historical geology first proposed by Nicolaus Steno in 1669 with the publication of De Solido. In De Solido, Steno compared his geological history of Tuscany with the biblical account of the Creation but did not acknowledge the possibility that the six-thousand-year biblical time scale might need to be expanded. Many modern commentators have suggested that the conflict between deep and biblical time represented a quandary for Steno, but evidence from the text of De Solido indicates Steno was more concerned that even the short biblical chronology would stretch the credulity of his readers. In particular, he was concerned that readers might find the preservation of fossil shells for even a few millennia implausible, and that they would be inclined to question the reliability of ancient reports that described geological change. More important threats to faith included the possibility that the world was eternal and that the biblical flood was merely a local event.