288 REPORT OF NATIONAL MUSEUM, 1892. 



fcioned, sedimentation was continuous and comparatively undisturbed 

 in others. Therefore, notwithstanding the occurrence of numerous and 

 extensive breaks, there lias been an abundant and continuous sedimen- 

 tary record made for the whole earth. It is to this continuous record 

 that the great geological scale is applied. Although the statement that 

 sedimentation has been continuous upon the larger part of the earth's 

 surface during every epoch of geological time now accords with the 

 views of all geologists, it will be seen by proposition 3, on page 291 that 

 in certain particulars it does not agree with those of the early geolo- 

 gists; but this matter will be discussed on following pages. 



In dividing and subdividing the scale geologists have generally rec- 

 ognized three grades of divisions, as is shown by the three columns of 

 names in each of the two preceding tables, the second grade being 

 subordinate to the first, and the third to the second. The divisions of 

 the hist grade are general, only three in number, and evidently very 

 unequal as to the actual duration of time represented by each. Those 

 of the second grade are more, and those of the third still more special. 

 The divisions of the third grade may each be locally represented by a 

 single formation or by more than one. That is, formations are not the 

 natural units of this scale. 



The different divisions of each of these three grades are unequal to 

 one another, but for convenience of classification they are treated as 

 of similar rank in each grade. It is not, however, to be understood 

 that the different divisions of the same grade represent at best more 

 than a remote approximation to equal portions of time or to equal 

 average advances of animals and plants in biographical rank Besides 

 this, the sedimentary accumulations which represent any one of the 

 divisions of the respective grades may be many times greater than 

 those of another division of the same grade, but the bulk of formations 

 is not an index of the relative length of time within which each was 

 accumulated, because the rate of sedimentary accunmltion was always 

 extremely variable. 



In discussing the divisions of such a classification as these tables 

 represent it is desirable that for the sake of clearness of statement each 

 of them should have both a structural and chronological designation. 

 That is, the general designation given to each of the assemblages of 

 strata which constitutes a division or subdivision in such a classification 

 ought, if practicable, to have a time correlative. The general failure of 

 authors to agree upon such a plan is doubtless due to the natural dif- 

 culty of correlating a chronological idea with physical objects which 

 among themselves are of unequal quantity and, to a certain extent, of 

 variable quality. In my own writings, however, I have generally used 

 the term epoch as a time correlative of formation, and the term age as a 

 time correlative of system,* but for present convenience I shall vary my 

 custom in the latter respect which will be the less objectionable because 



See for example, Geology of Iowa, White, 1870, vol. i, p. 25. 



