Trenton Limestone in Iowa

Of late, stratigraphic taxonomy and correlation take on new trend from that to which the last generation is accustomed. They neither go to the paleontological extreme in which the life-zones are regarded as passing around the globe like the successive skins of an onion, nor do they severely restrict themselves to the lithologic, or mapping unit extreme, in which faunas are largely, if not entirely, ignored. As in so many cases in science generally the truth really lies somewhere midway between the extremes. But the new trend has altogether different basis from that presumed for the old. It is diastatic in character. So that some of our long recognized geological formations manifestly have much wider expanse, and others very much more restricted distribution than latterly it is the disposition to assign to them