G. K. Gilbert — Congress of Geologists. 439 



Jurassic. It is a natural system in Europe. In the eastern 

 United States no strata are called Jurassic with confidence, and 

 at the west the rocks called Jurassic merge with those called 

 Triassic. In India, Medlicott tells us, a Jurassic fauna occurs 

 at the summit of a great natural system containing a Permian 

 fauna near its base.* In New Zealand, according to Hutton, a 

 continuous rock system, dissevered by great unconformities 

 from other systems, bears at top fossils resembling those of the 

 lower Jurassic and lower down fossils of Triassic facies.f To 

 establish a Jurassic system in either of these countries it is 

 necessary to divide a natural system, and a Jurassic system thus 

 established would be necessarily artificial. 



This is the sort of classification implied by the assumption 

 that systems are world-wide. It, is not impossible, but it is 

 highly unadvisable. It is classification for the sake of uni- 

 formity, and its uniformity is procrustean. The natural systems 

 of a region are the logical chapters of its geologic history. If 

 you group its strata artificially according to the natural divis- 

 ions of another region, you mask and falsify its history. The 

 geologic history of the earth has as great local diversity as its 

 human history. As in human history there are interrelations 

 and harmonies and a universal progress, but these are percepti- 

 ble only in the general view ; and the student whose preconcep- 

 tions lead him to exaggerate the harmonies and ignore the dis- 

 crepancies perverts the meaning of every page. 



I prefer therefore my own definition of system, making it 

 natural and consequently local, and I earnestly oppose any at- 

 tempt to coerce the geology of one country in a rigid matrix 

 formed over and shaped by the geology of another country. 



The ideas I oppose have arisen in connection with the work of 

 correlation. Some geologists appear to regard correlation as the 

 determination in distant localities of identities; the more philo- 

 sophic regard it as the determination of the actual relations, 

 whether they be of identity or difference. With the former the 

 basis of correlation is the universality of geologic systems ; with 

 the latter it may be said to be the universality of geologic time. 



Now in the comparative study of local geologic histories, just 

 as in the comparative study of local human histories, it is a 

 matter of convenience to have a common scale of time. It is 

 not essential, but it is highly convenient. In human history 

 we use an astronomic scale of equal parts, designating each 

 unit by a number. In geology no scale of equal parts is avail- 

 able, and we employ the eras and periods, and to some extent 



* Sur le coloriage des Cartes geologiques, par M. H. B. Medlicott ; in Congres geolo- 

 gique international, Compte rendu de la 2me session, Bologne, J 881, pp. 652-656. 



f Sketch of the Geology of New Zealand. By Captain F. W~. Hutton. In 

 Quart. Jour. Geol. Soc. London, vol. xli, pp. 191-220. 



