Kent Dennis V. author Columbia University. Lamont-Doherty Earth Observatory Columbia University. Lamont-Doherty Earth Observatory originator text Letters 1981 English A distinguishing feature of the asteroid impact hypothesis presented by Alvarez et al. (6 June, p. 1095) for the end-of-Cretaceous biotic crisis is that it is based on direct physical evidence: the distribution of iridium in several Cretaceous-Tertiary sedimentary sections. I draw attention to other evidence which suggests that (i) a high Ir concentration may not be uniquely associated with an extraordinary extraterrestrial event and (ii) the impact of a large asteroid in any case is not likely to have had the dire consequences to life on the earth that they propose. Geochemistry 211 4483 648 650 1981-02-13 http://dx.doi.org/10.1126/science.211.4483.648 http://hdl.handle.net/10022/AC:P:11959 NNC NNC 2011-12-12 14:31:37 -0500 2013-05-13 12:39:52 -0400 5946 eng