I40 The American Geologist. March, i904. 
rules liiey are classed as (a) sedimentary, (b) igneous, and 
(c) nietamorpliic. Sedimentary is used in a broad sense, to in- 
clude rocks formed by aqueous, organic, glacial, and eolian 
agencies. ■ The word "formation," which under the earlier rules 
was practically restricted to structural divisions of the fossil- 
iferous elastics, is now applied to rocks of all classes. It is the 
cartographic unit or individual, and is ordinarily the smallest 
rock body separately named and mapped. Exceptionally, and 
chiefly in connection with economic work, smaller bodies, called 
members, are represented and named. 
Under the earlier rules the sedimentary formation was de- 
fined by its physical characters alone, its fossil contents being 
used only for purposes of correlation. Under the new rules its 
definition may depend largely on contained fossils. 
In the classification of formations, the earlier rules specified 
but one category — the period — thus giving fixed taxonomic 
rank to only two words — "formation" and "period." The new 
rules substitute (in the geologic atlas) the corresponding rock 
term "system" for the time term "period," and give definite 
. rank to intermediate terms. Arranged according to rank, and 
beginning with the lowest, their scheme of stratigraphic divis- 
ions is member, formation, group, scries, system. Members, 
formations and groups are local or provincial, series are pro- 
vincial or broader, systems are world-wide. 
As will be seen by the subjoined table. Pleistocene, Neo- 
cene, and Eocene are abandoned as primary classific terms, and 
Quaternary and Tertiary are re-established. Pleistocene is 
given series rank, to include the deposits of the glacial period, 
and return is made to the Lyellian divisions of the Tertiary. 
The compound Jura-Trias is abandoned ; Permian, Pennsyl- 
vanian, and Mississippian are recognized as series of the Car- 
boniferous ; Ordovician is accepted in place of Lower Silurian, 
and given rank as a system ; and Saratogan, Acadian, and 
Georgian are adopted as series of the Cambrian. Algonkian 
and Archean are redefined in such way as to recognize the pres- 
ence of metamorphic elastics among the dominantly igneous 
rocks of the Archean. 
