142 
IOWA ACADEMY OP 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, f 
recognizing the fact that exact synchrony could not be 
established by means of fossils, proposed the term, homotaxis, 
indicating similarity and not time-equivalency of organic con- 
tents. Irving,]: 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 versa ; that fossil facies represent the product of two prin- 
cipal factors, of which one (environment) is so variable, 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. To-day the larger groups are 
confidently 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, ” Walcottl 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. 
tQuart. Jour. Geol. Soc. London, Vol. XVIII, p. 14, 1863. 
$U. S. Geol. Sur., 7th Ann. Kept., pp. 365-454, 1888. 
§U. S. Geol. Sur., Bull. 86, pp. 511-534, 1893. 
II Am. Jour. Sci., (3), Vol. XL, p. 36, 1890. 
*lfU. S. Geol. Surv., Bull. 81, p. 433, 1891. 
