But we have not yet done with this part of our investigation. The argument from geology is based on the assumption that the chronological order of the earth's layers _has been determined_ at least with great approximation to certainty, so that we may say with some assurance that this layer of limestone or sandstone is of earlier, that, of later origin. As a matter of fact, the textbooks do treat the various "ages" of geology as if they corresponded to certain strata of the earth's crust. _But by what method is the age of the various layers determined?_ James D. Dana in his "Manual of Geology" (Fourth edition, p. 398 f.) says that there are four methods by which we may decide the relation of one layer to another. The first is, naturally, the order in which the layers rest upon one another; the lower strata, are, of course, older than the upper. However, he points out in four "precautions" the inability of the investigator to depend on this method, since "for the comparing of rocks of disconnected regions, this criterion must fail." Also the color and mineral composition can be used only "with distrust" and must be "usually disregarded." Then the _Manual_ proceeds: "4 _Fossils_.--The criterion for determining the chronological order of strata dependent on kinds of fossils takes direct hold upon time, and therefore, _is the best;_ and, moreover, it serves for the correlation of rocks all over the world." Now observe how, in the following, the geologist leans upon the evolutionist: _"The life of the globe has changed with the progress of time. Each epoch has had its peculiar species, or peculiar groups of species._ Moreover, the succession of life has followed a grand law of progress, involving under a single system a closer and closer approximation in the species, as time moved on, to those which now exist. It follows, therefore, that _identity of species of fossils proves approximate identity of age."_ Let us bear this in mind. Dana _takes for granted_ the evolutionary process. The simpler forms of animal life indicate the older strata, the complex forms, the more recent. We do not misunderstand Mr. Dana. Such expressions as the following abound: "Where direct paleontological observation has ascertained in particular cases the steps of progress in the development of organs, as, for example, those of the teeth in Mammals, the facts become a basis for further use in the same direction." (p. 402.) "The grander divisions of geological time should be based, in a comprehensive way, on organic progress" (from simple to more complex structures) (p. 404.) "When the relations of the beds to those recognized in other regions have been ascertained through fossils..." (p. 405.)