Chiasmus and Chaistic Structure

 A chiasmus is a literary form in which the first part is repeated in the second part but in reverse:

• "Ask not what your country can do for you; ask what you can do for your country" (Speech by President Kennedy)
• "Fair is foul and foul is fair" (Macbeth)

Some narratives employ this structure. For example, in the story of Noah's Ark in Genesis (which has an overall chiastic structure) the timings are as follows:

• Seven days warning during which Noah collects all his animals and enters the Ark
• Seven days before the flood proper starts
• Forty days (and forty nights) of rain.
• One hundred and fifty days floating about in the Ark.
• God 'has mind of' Noah
• One hundred and fifty days while the waters decrease and the mountain tops reappear (and the Ark gets grounded on Ararat
• Forty days further before Noah looks out to see what is happening and sends out a dove which returns
• Seven days waiting before he sends out a second dove which comes back with a twig in her beak
• Another seven days waiting, just to be on the safe side, before Noah leaves the Ark.

Some people believe that the Bible is the Word of God so the record of the days is literally true. But if you analyse it as a literary text it seems likely that the narrative was tweaked to fit the 7,7,40,150,150,40,7,7 structure.