()^ GEOLOGICAL BIOLOGY. 



thus become independent sources of information regarding time- 

 succession. From this point of view we find fossils to be the 

 marks of the stages of progress in Hfe-histories upon the earth, 

 and the strata then serve, as the sand in the hour-glass, to 

 m^easure the length of the time-intervals spanned by the life of 

 particular species, genera, or families, or of faunas or floras. 



The Necessity of Two Scales; Strata Furnishing the Data for the 

 Formation-scale and Fossils Forming the Basis of the Time-scale. 

 • — This new point of view will lead to the separation ot the 

 time-scale from the formation-scale, and the making of a dual 

 nomenclature and classification. The fossils, independent of 

 the thickness or succession of the strata holding them, have a 

 definite time-value, as indicated by the classification of the 

 scale into Paleozoic, Mesozoic, and Cenozoic times, and the 

 Eocene, Miocene, and Pliocene divisions of the Tertiary, pro- 

 posed by Lyell. 



The extension of this method of dividing the time-scale 

 results in the formation of a pure time-scale, based upon the 

 stages in the life-history of the fossil faunas, quite independ- 

 ent of the formations of any particular section, although 

 adopting the systems, arbitrarily, as well-known and recog- 

 nized units of universal distribution.* 



Use of the Terms Period and Formation — In treating of his- 

 torical geology we speak of the age of invertebrates, the age 

 of fishes, the age of coal plants, etc., but the application of 

 time-designations to the rocks or formations is always per- 

 plexing and often leads to confusion of ideas. The terms 

 Silurian, Cretaceous, Permian, Trenton, or Miocene were 

 -names of rock formations before they could be applied to the 

 periods of time in which the formations were made. This 

 double usage was introduced as early, at least, as 1828, when 

 Lyell proposed to divide the Tertiary formation into " four 

 groups or periods to which they belonged," calling them 

 Eocene, Miocene, older Pliocene, and newer Pliocene. Al- 

 though the science demands two classes of designations, a 

 time-scale and a formation-scale, it certainly will tend to 



* See p. 52 also " Dual Nomenclature in Geological Classification, "_/o?<;'««^ 

 of Geology, vol. ii., February-March, 1894, pp. 145-160. 



