STRATIGRAPHIC TAXONOMY 597 



divided. It is perhaps imnecessary to state that at certain times they are 

 included in the operation of the maximnm movements. 



Finally, there were many minor movements, under which term we 

 may refer to tilting of land masses, whether these involved the greater 

 part of a continent or only some subordinate ^^positive" part of same, 

 and to all movements that are relatively local in their effects on the 

 strandline. Theoretically, these minor movements define geological for- 

 mations or ages; also groups of formations which are distinguished on 

 lithological grounds, but seem to have been deposited without break. 

 They are commonly marked by retreat and subsequent advance of the 

 sea without decided change in provincial boundaries, by deepening or 

 emergence, as the case may be, of the subsidiary troughs of a geosyncline, 

 and by local changes in the lithological facies of formations. 



Obviously there is no sharp distinction between these three grades of 

 movements. The minor movements vary to the point where they may 

 justly be called major, and these again grade into those of the highest 

 or maximum rank. Their taxonomic value, therefore, is correspondingly 

 indefinite and peculiarly liable to personal differences in their estimation. 

 The most generally admitted, if not the most convincing, part of the 

 evidence from which diastrophic movements are inferred, namely, the 

 clastic deposits, varies so greatly from place to place that wide difference 

 of opinion respecting the taxonomic significance of the several occur- 

 rences is to be expected. The theory of inland migration of belts of 

 active folding, also the proved small relief of the interior areas of 

 the continents, helps greatly in explaining the paucity of detrital de- 

 posits at localities and times when much more might be expected. For- 

 tunately, we have two other classes of evidence — (1) that showing strati- 

 graphic overlaps, and (2) that showing the geographic derivation and 

 distribution of fossil faunas and floras — that give no less convincing 

 testimony in proving the frequency and varied kinds of movements. 

 These two classes of evidence bear directly on the displacements of the 

 strandline, and the variations in direction and extent of its transgressions 

 give the surest proof of the instability of the lithosphere. Further, since 

 the rhythmic periodicity of diastrophic processes is almost universally 

 accepted as a fundamental fact in geologic science, these displacements 

 logically become not only the ultimate but also the immediate natural 

 basis of exact stratigraphic taxonomy. 



Chamherlin and Sali^'biiri/s views on the natural basis of time divis 

 ion. — There are geologists who deny that the divisions of geologic time 

 which are natural for one res^ion are similarlv natural for another re- 



