440 G. K. Gilbert — Congress of Geologists. 



the epochs, of the local geologic history first deciphered — that 

 of Europe. These time divisions bear the same names as the 

 groups, systems and series of strata whose deposition occurred 

 within them. 



So far as the science of geology is concerned the selection of 

 Europe as its first field of study was a matter of chance, and 

 the adoption of the European time scale as a general standard 

 may therefore be said to have been accidental. Though the 

 local rock scheme on which it is based is natural, the time scale,, 

 considered as universal, is arbitrary. Another locality would 

 have afforded a different scale, but its authority would neither 

 be greater or less. The scale being recognized as arbitrary, 

 and a mere matter of convenience, it is legitimate to modify 

 and fix it by formal convention. The Congress can do good 

 service to geologic technology by putting it in the best possible 

 shape and giving it an official status. In my judgment only a 

 small number of divisions should be admitted, not more than 

 the number of periods of the European scheme. In a gen- 

 eral way the durations represented by the coordinate divisions 

 should be as nearly equal as practicable, but a certain con- 

 cession might be made to chronologic perspective on account of 

 our superior opportunities for studying the later history. 

 Some of the shorter periods might perhaps be united under 

 new names. Each line of division between periods should 

 be defined by means of a stratigraphic plane of division, 

 and this can be done with precision if a locality is made part of 

 the definition. 



Especially should pains be taken to declare the arbitrary 

 nature of the scale. Even with this precaution it will be mis- 

 construed by many, for there is a tendency of the mind to 

 attach undue weight to classification. Wherever we draw lines 

 of separation we lose to a certain extent the power to recog- 

 nize continuity. When, for example, the clock strikes twelve 

 on New Year's Eve time seems to stop and begin again. We 

 speak of the achievements of the nineteenth century — and 

 despite ourselves we think of them, too — as though a new 

 industrial epoch began in A. D. 1800. And so it is easy for 

 the beginner in geology to accept as discontinuous the eras and 

 periods of which his text-book treats, and it is hard for him 

 afterward to unlearn the lesson. 



There is reason to believe that confusion of ideas in regard 

 to geologic classification has been fostered by the employment 

 of the same set of names for the divisions of the time scale and 

 for the local terranes on which they are founded. It might be 

 well to furnish the time scale with names suggesting time — such 

 names as the brothers Rogers applied to the terranes of Penn- 

 sylvania — but so radical a change is hardly feasible, especially 



