Nov. 3, 1887] 



NATURE 



2.1 



abroad, it is proper that in seeking the true one we begin with 

 the elements. 



The surface of the land is constantly degraded by erosion, and 

 the material removed is spread on the floor of the ocean, form- 

 ing a deposit. This process has gone on from the dawn of 

 geologic history, but the positions and boundaries of land and 

 ocean have not remained the same. Crust movements have 

 caused the submergence of land, and the emergence of ocean 

 bottom, and these movements have been local and irregular, dis- 

 tricts here and there going up while other districts went down. 

 The emergence of ocean bottom exposes the deposit previously 

 made on it, and subjects it to erosion. In this way every part of 

 the known surface of the globe has been the scene of successive 

 deposition and erosion, and in many districts the alternations of 

 process have been numerous. It is manifestly impossible that 

 either erosion or deposition should ever have prevailed univers- 

 ally, and it has been established by the study of stratigraphic 

 breaks that a time of erosion has often interrupted deposition in 

 one region while deposition was uninterrupted in another. 



In transportation from its region of erosion to its place of 

 deposition detritus is assorted, and it results that the simulta- 

 neous deposits on the bottom of an ocean are not everywhere the 

 same. Equal diversity is shown in the ancient deposits consti- 

 tuting geologic formations. It is a general fact that synchronous 

 formations have not everywhere the same constitution. 



Many of the variations in deposits are correlated with depth 

 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- 

 t^-ibuted, 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 Palteozoic 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 

 provinces 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 revolu- 

 tion of land areas, leads to two antagonistic tendencies, one 

 toward diversity of life on different parts of the globe, the other 

 toward its uniformity. 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 pro- 

 cesses it has acqiyred a stratigraphic constitution ot 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 difterent, 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 separated 

 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 unconformi- 

 ties 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 impossible, 

 and in many cases the gradation of a terrane as a group, a sys- 

 tem, or a series is largely a matter of convenience. From this 

 point of view a system is somewhat artificial, but there is a more 

 important sense in which it is natural. It is limited by strati- 

 graphic or palasontologic breaks above and below, and these 

 breaks are natural. The taxonomist is not warranted in dividing 

 systems where no such break exists. 



Transferring now our attention to some other area, distant 



from the first, and studying its stratigraphy, we find that the 

 same principles enable us to divide it independently into stages, 

 series, systems, and groups. Its fossils are not the same, but 

 they are to a certain extent similar, and the sequence of life is 

 approximately parallel. We cannot compare stage with stage, 

 nor series with series perhaps, but we can compare system with 

 system, and making the comparison we discover that the breaks 

 are at different places. While one area was upraised and sub- 

 jected for a time to erosion, the other received continuous 

 deposition. While life in one area, enjoying constant condi- 

 tions, was almost unchanged for long ages and even epochs, it 

 was revolutionized in the other by the irruption across some 

 obsolescent barrier of strong and aggressive faunas and floras. 

 The systems of one area, therefore, do not coincide with the 

 systems of the other in their beginning and ending. They may 

 differ in number, and they may differ greatly in magnitude, and 

 in the duration they represent. They are, in fact, a different 

 set of systems. 



The case I have described is ideal, but not false. It repre- 

 sents the common experience of those who have developed the 

 geologic histories of remote districts, and attempted to correlate 

 them with the geologic history of Europe. There does not 

 exist a world-wide system nor a world-wide group, but every 

 system and every group is local. The classification developed 

 in one place is perfectly applicable only there. At a short 

 distance away some of its beds disappear and others are intro- 

 duced ; further on, its stages cannot be recognized ; then its 

 series fail, and finally its systems and its groups. 



If I have properly characterized stratigraphic systems — if 

 they are both natural and local — it goes without saying that the 

 classification of the strata of all countries in the dozen or so 

 systems, as proposed by some of the members of the Congress, 

 is impossible. 



I hasten to add that from . the point of view of these gentle- 

 men what they advocate is not necessarily impossible, for they 

 have a different conception of a system. They regard it not as 

 local but as universal. It is their privilege to define their terms 

 as they please, and we will not dispute about mere words, but I 

 cannot too strongly or too earnestly insist that a system which is 

 universal is artificial. It may be natural in one geologic 

 province, but it is artificial in all others. Take for example the 

 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 Permiaa 

 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. 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 divisions 

 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 inter-relations and 

 harmonies and a universal progress, but these are perceptible 

 only in the general view, and the student whose preconcev tions 

 lead him to exaggerate the harmonies and ignore the discre- 

 pancies 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 

 attempt 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 



