wiluamb.] LETTER OF TRANSMITTAL. 9 



impossibility, of using in all countries the same set of names for stratigraphic divi- 

 sions smaller than systems. 1 



By comparing the list of geologic systems in this " plan " with the 

 list in the passages cited from the report of the Director, it will be 

 observed that there are slight discrepancies. The unsettled problems 

 of nomenclature thus suggested were elaborately discussed by a con- 

 ference of the geologists of the Survey held in January, 1889, for the 

 purpose of establishing the conventions necessary to uniformity in the 

 preparation of the sheets of the Geologic Atlas of the United States. 

 3y that conference it was determined that the stratigraphic units de- 

 lineated on the sheets of the geologic atlas should be designated as 

 formations, that no stratigraphic unit of a higher order should be rec- 

 ognized in the atlas, and that the only term of classification there em- 

 ployed should be the geologic " period." 2 The time-term " period " thus 

 adopted for the geologic atlas has the same taxonomic rank as the strati- 

 graphic term " system " employed in the " plan n for the instruction of 

 the essayists and in the passage cited from the report of the Director. It 

 was preferred by the geologists of the conference because it was believed 

 that the major classification expressed by either term is essentially arbi- 

 trary and does not find in nature a universal expression, either physi- 

 cally through lithologic and structural differences, orbiotically through 

 the differentiation of faunas and floras. The chronologic term seemed 

 to them freer than the stratigraphic from the implication that the 

 classific units are natural and general rather than artificial or local. 



The conference likewise indicated and defined eleven periods to be 

 used in the classification of the formations represented in the atlas, and 

 designated them as follows: (1) Pleistocene, (2) Neocene, (3) Eocene, 

 (4) Cretaceous, (5) Jura-trias, (G) Carboniferous, (7) Devonian, (8) Silu- 

 rian, (9) Cambrian, (10) Algonkian, (11) Archean. 1 These are the exact 

 equivalents of the "systems" enumerated in the preceding quotations, 

 but they differ somewhat as to name. 



The conventions thus adopted for the work of the Geological Survey 

 have modified and controlled the work of the division so far as they 

 are applicable, and the substitution of "period" for "system" has 

 changed the point of view of the essays in a manner conducive to their 

 simplification and to their value as contributions to the subject of cor- 

 relation. 



Although the essayists, working under the same general instructions, 

 have had before them the accomplishment of the same purposes with 

 respect to 'the several groups of formations assigned them, no attempt 

 has been made to mold their modes of treatment in a common form. 



1 This plan was published in the Tenth Annual Report of the Survey as part of a progress report of 

 the work of the Division of Correlation (pp. 108-113). Further report of progress may be found in the 

 Eleventh Annual Report, pp. 59-62. 



2 Tenth Annual Report U. S. Geological Survey, pp. 63-65. 

 *Ibid., pp. 65-66. 



