I read this English translation in the late-1970s/early 1980s.
All I can remember is the opening: the Catholic Church decides to comb through its religious relics to determine which ones are genuine. They discover a number of 'relics' are not genuine - one, which has been displayed in a church since the Middle Ages, is clearly a 20th-Century pilot's oxygen mask.
A number of other temporal anomalies are discovered, leading people to theorize that time travel is possible. It is possible; a secret government time-travel project already exists (I can't remember if it was U.S. or French & I can't remember what its purpose was).
I also learned that the Mediterranean Sea has periodically been an 'evaporation basin'; if it were cut off from the Atlantic Ocean it would be a desert & a civilization once existed there.
The book started off strong but ended poorly, but I would like to read it again.