SEDIMENTATION, ENVIRONMENT, AND EVOLUTION. 9 



result of differing environment, a strong tendency will exist 

 for a, a', a" , etc., to be grouped together on account of litho- 

 logical similarity. Such an occurrence is almost inevitable if 

 the deposits are in due course mapped geologically, for the 

 time-planes XX, YY are difficult to follow, and the planes 

 aa, ft ft become more strongly developed because of the varia- 

 tion in lithology above and below them ; they are, in fact, 

 the stratigraphical planes or bedding-planes as commonly 

 known, planes which are usually and tacitly assumed to be 

 true time-planes, but which actually cut across the latter. 

 Only under exceptional conditions of fossil preservation, and 

 then by accurate and detailed zonal study alone, can the true 

 contemporaneity of deposits be determined. 



The complementary case of a rising shore is represented 

 ideally in Fig. 2, where the deposits are seen to be thrown 

 successively seawards as elevation occurs. Again the litho- 

 logical planes aa, ft ft cut across the true time-planes XX, YY, 

 and the deposits a, b, c, and d, and a', b', c', and d', are 

 respectively contemporaneous. After subsidence, " interform- 

 ational conglomerates," such as that bounded by aa and ft ft, 

 may be formed. 



In plan, the effects of this discordance between the two 

 sets of planes would appear as Fig. 3, where the letters have 

 the same significance as in Figs. 1 and 2. Since topography, 

 drainage, soil, and vegetation are largely dependent upon the 

 lithological character of the underlying rocks, the lines repre- 

 senting the traces of the planes ftft, 77 would probably be 

 mapped as the " boundaries " of the " formation." The time- 

 planes, revealed by the contained fossils, would run obliquely 

 through the " formation " as XX, YY, etc. (the broken lines). 

 Nevertheless, the planes /3/3, 77 are actually parallel to the 

 shore-line, whilst XX, YY are not. 



From Figs. 1 and 2, it is evident that the steeper the shore, 

 the greater will be the discordance between aa, ft/3, 77, etc., 



