There is a theory that the iridium found in some rock layers may have originally come from meteorites. This is called the Alvarez Theory, and was proposed by Walter Alvarez in the 1970s.
Iridium is very rare in the earth's crust and occurs at a higher concentration in meteorites. Alvarez discovered a rock layer that contained about 30 times the concentration of iridium normally found in the earth's crust. The age of the layer indicates that the iridium was incorporated near the end of the Cretaceous Period. The amount of iridium was consistent with a large metereorite impact, and other iridium deposits were subsequently discovered around the world suggesting that many meteors struck the earth during that time period. This provides evidence to support the theory that the extinction of dinosaurs at the end of the Cretaceous Period was caused by a world-wide meteor event.