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IOWA ACADEMY OF SCIENCE 
gic time. At B, where the biologic section is made the point of special inquiry, 
the recurrence of faunas is graphically explained. More important than this is 
the weakness of the biotic method of geologic correlation that is shown; and the 
great strength and exactness of the correlative methods which are purely 
physical in character that are indicated. 
The sedimentative section with its contained fossils stands for the con- 
tinuous record; while the erosive history represents the rhythmic breaks which 
make possible exact correlation of terranes and general stratigraphic classifica- 
tion. The stratigraphic expression of the latter is the unconformity. This 
again is the outcome of diastatic, or diastrophic, movements, or more impres- 
sively and more readily recognizable in the field, the results of mountain-mak- 
ing changes upon the position of the shore-line of the ocean. Systematic ar- 
rangement of terranes on this basis is fundamental; it is strictly genetic; it is 
not dependent upon the often more or less fanciful interpretation of fossils; 
it is directly in harmony with the laws controlling sedimentation inself; it is 
the most practical and exact of any method yet devised; and it enables the 
votaries of geologic correlation to swing entirely clear of paleontology. 
The nicety and rapidity with which the orotaxial principles act in practice 
are indicated by a number of concrete examples. In the Upper Mississippi 
valley the values of the different methods of geologic correlation have been 
recently specifically compared.* 
In the Ozark region the shortcomings of the older methods of geologic 
classification have been pointed out.t Around the southern end of the Rocky 
mountains, in central New Mexico, the great value of the orotaxial method has 
been especially emphasized.! Its value has been determined in the unfossil- 
iferous Tertiary deposits of the Death Valley region in eastern California and 
Nevada.! Earlier Irving** strictly followed the method in correlating the 
Pre-Cambrian sequence of the Lake Superior region; and McGeett applied its 
principles to the unfossiliferous formations of the Atlantic Coastal plain. 
In the present advanced state of stratigraphical science, in which recon- 
naisance work is no longer needful over the large part of our country, it seems 
that we have reached a stage where classification of terranes should follow 
definite principles in accordance with the taxonomic ranks of the various 
geologic units, much in the same way that it is accomplished in botany or 
zoology. A dual geologic classification — one structural and the other biotic — 
is certainly superfluous. The biotic scheme may be advantageously eliminated 
entirely as it is now really done in practice by all except the old-school pale- 
ontologists. 
We may arbitrarily recognize the larger divisions as worldwide time-divi- 
sions; and regard the sediments as deposited during certain eras or periods. 
The latter may also be advantageously subdivided into Early, Mid and Late 
classes, still retaining the time criterion. Below the taxonomic rank of period, 
or sub-period, however, geologic sections are provincial in character. The 
structure sequence of the region now becomes the most critical of the corre- 
*Proc. Iowa Acad. Sci., Vol. X, pp. 105-107, 1903. 
tBull. Geol. Soc. America, Vol. XII, pp. 173-196, 1901 ; also, Ibid., Vol. XIII, pp. 267- 
292, 1902. 
!Am. Jour. Sci. (4), Vol. XXI, pp. 296-300, 1906. 
§Trans. American Inst. Mining Eng., Bull. No. 34, pp. 867-903, 190'9. 
**U. S. Geol. Surv., 7th Ann. Kept., pp. 437-439, 1888. 
tfCong. geol. international, 5me Sess., p. 164, 1903. 
