G. K. Gilbert — Congress of Geologists. Wl 



of water and distance from shore, and it results that elevation 

 and subsidence in regions of continuous deposition produce 

 changes in the nature of the local deposit. 



The animals and plants of the earth are not universally dis- 

 tributed, but are grouped in provinces. In the geologic past 

 similar provinces existed, but their boundaries were different, 

 shifting in harmony with the varying geography of the surface. 

 From time to time the barriers separating contiguous provinces 

 have been abolished, suffering them to coalesce ; and conversely 

 new barriers have arisen, creating new provinces. From the 

 earliest paleozoic to the present time the species of animals and 

 plants have been progressively modified, the nature of the modi- 

 fication depending on local conditions. The faunas and floras of 

 different provinces thus become different, and the longer the pro- 

 vinces remain distinct, the greater is the divergence of life. The 

 removal of a barrier either produces a new fauna by the fusion of 

 the two previously separated, or else obliterates one and extends 

 the area of the other. In either case there is a change toward 

 the unification of life, and in either case there is an abrupt 

 change in a local fauna. Thus the secular evolution of species, 

 combined with the secular and kaleidoscopic revolution of land 

 areas, leads to two antagonistic tendencies, one toward diversity 

 of life on different parts of the globe, the other toward its uni- 

 formity. The tendency toward uniformity affords the basis for 

 the correlation of terranes by comparison of fossils ; the tendency 

 toward diversity limits the possibilities of correlation. 



If now we direct attention to some limited area and study its 

 geology, we find that under the operation of these general proc- 

 esses it has acquired a stratigraphic constitution of a complex 

 nature. Its successive terranes are varied in texture. Breaks 

 in the continuity of deposition are marked by unconformities. 

 The fossils at different horizons are different, and when they are 

 examined in order from the lowest to the highest, the rate of 

 change is found to vary, being in places nearly imperceptible 

 and elsewhere abrupt. It is by means of such features as these — 

 that is, by lithologic changes, by unconformities, and by life 

 changes — that the stratigraphic column is classified into groups, 

 systems, series and stages. A system is a great terrane separ- 

 ated from terranes above and below by great unconformities, or 

 great life breaks, or both. Smaller unconformities, smaller life 

 changes, and lithologic changes are used for the demarcation of 

 series and stages; and on the other hand, exceptionally great 

 unconformities and life breaks are used to delimit groups. As 

 the same criteria determine groups, systems and series, differing 

 only in degree, the precise definition of the term system is im- 

 possible, and in many cases the gradation of a terrane as a 

 group, a system or a series is largely a matter of convenience. 



