Molecular clouds and periodic events in the geologic past

The suggestion that a claimed 30 Myr period in the geologic past resulted from cometary impacts following encounters with molecular clouds as the solar system oscillates about the galactic plane poses a well-defined problem in the theory of shot noise. All recent CO surveys of the Galaxy clearly indicate that the concentration of molecular clouds in the galactic plane is not sufficient to allow a statistically significant period to be extracted from the small number of dated events. Of the order of 1000 events is probably required to obtain a credible period.