Where It Is Difficult To Distinguish
Another paper written by Richard Monasterky gives more evidence. It seems that a scientists named Gregory J. Retallack went searching in the Southern Hemisphere and reported that he found shocked quartz at two sites in the Antarctica and one site in Australia. This type of quartz is riddled with intersecting sets of fractures and is born only during impacts. Iridium also adds to the evidence. Scientists now know that an impact caused the extinction at the Cretaceous-Tertiary boundary and there is also an increase in Iridium at this boundary.
Thus they can deduce that if they find and increase of iridium at the PTB above background levels then they will have evidence of an impact. The discovery of a significant increase in iridium and microspherules at the PTB boundary by Xu Dao-Yi et al. (1993, 1985, 1989) gives imperical evidence of an impact.There are many problems with the impact theory.
First there is evidence that shows that the Permian extinction started gradually and had a more rapid pulse at the end (Monastersky, 1993). Also some scientists have argued that the quartz crystals found by Retallack were not shocked because Retallack only studied them under a light microscope, where it is difficult to distinguish shock features from more prosaic deformations caused by normal tectonic stress in the Earths crust.
