1.   Mixed in with the global iridium-bearing clay layer are tiny particles of heavily shocked quartz.

2.   Shocked quartz has been found all over Earth, but the largest fragments exist in North America.

3.   Study of the samples with light and electron microscopes revealed multiple signatures of shocked quartz, as well as glassy areas that implied a history of fiery melts.

4.   He said he would be more convinced by shocked quartz grains, an additional sign of an extraterrestrial impact.