142 IOWA ACADEMY OF SCIENCES. 



Smith* explained a method whereby the different strata could 

 be recognized by the fossils which they contained, organic 

 remains have been the foundation of all geological classifica- 

 tions. Of late years, when other methods have been devised, 

 numerous discrepancies have arisen between the conclusions to 

 be deduced from two sets of facts, and the question has begun 

 to arise on all sides as to just how far the fossils can be relied 

 upon in the correlation of geological formations. Huxley, t 

 recognizing the fact that exact synchrony could not be 

 established by means of fossils, projjosed the term, homotaxis, 

 indicating similarity and not time-equivalency of organic con- 

 tents. Irving, J and later Van Hise,§ and others, working 

 in very ancient, non-fossiliferous rocks were obliged to swing 

 loose altogether from the use of organic remains. McGee, || in 

 discussing the subject, concludes that, in correlating by means 

 of fossils, ' it is the weakness of the method that many 

 rocks are too poor in fossils to be correlated thereby; that 

 formations may be homotaxial yet not contemporaneous, and 

 vice verm\ that fossil facies represent the product of two prin- 

 cipal factors, of which one (environment) is so varia'^>le under 

 local conditions, that the product is inconstant among the 

 minor rock divisions, and that the geologic chronometers 

 afforded by fossil plants, fossil invertebrates, and fossil 

 vertebrates, respectively, give unlike time units and, some- 

 times discordant readings. Today the larger groups are 

 contidently correlated by paleontology; but leading American 

 geologists no longer accept identity of fossil facies as final 

 proof of equivalence among the minor rock divisions." 



In his correlation essay on the ' ' Cambrian of North Amer- 

 ica, " Walcott*! not only says, as already stated, that "for the 

 determination of synchrony, except in a limited area, there is 

 little hope for satisfactory conclusions by any methods yet 

 devised," but in referring to paleontology in particular, 

 remarks that "all paleontologic reasoning is based upon 

 known data. By the discovery of a new grouping of fossils, or 

 a different range of known species, the identification of 

 horizons may be materially modified. " As coming from the 



*Geol. Table British Org. Foss., 1815. 



■t-Quart. Jour. Geol. Sic. London, Vol. XVII 1, p. 14, 1862. 



*U. S. Geol. Sur., 7th Ann. Rept., pp. 365-451, 1888. 



§U. S. Geol. Sur., Bull. 86, pp. 511-534. 1892. 



II Am. Jour. Sci., (3). Vol. XL, p. 36, 1890. 



"U. S. Geol. Surv., Bull. 81, p. 423, 1891. 



