﻿Vol. 6 1.] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxV 



which may be ultimately utilized for the purpose of constructing a 

 time-scale, such as will not only enable him to detect events which 

 happened synchronously in different parts of the world, but also 

 will give him a clue to the actual lengths of the intervals of time 

 indicated by various lengths of the geograms. 



The principal variations in the records of these geograms are 

 due to alternate formation and cessation of deposit ; to differences 

 in character of the deposits, owing to various local conditions ; 

 to accumulation of contemporaneous volcanic materials ; to varia- 

 tions in the nature of the earth-movements ; to changes in the 

 nature of the included organisms ; and, lastly, to climatic changes. 

 Some of these are, of course, to some extent dependent upon the 

 others, but it will be convenient to deal with them separately. 



We may proceed to consider the significance of these records, as 

 bearing upon the classification of the sedimentary rocks. 



(1) Alternate Formation and Cessation of Deposit. 



Apart from the frequent existence of a plane of stratification 

 between dissimilar deposits, to which further reference will be 

 made, we frequently find two similar masses of deposit separated by 

 a plane of stratification marking a want of coherence between the 

 materials of the two masses. Such a plane of stratification is 

 generally recognized as denoting a period of cessation of deposit ; 

 and it is equally recognized that such cessation may be due to 

 various causes, such as shifting currents, alteration of the position 

 of main discharge of a river, or establishment of a river's base- 

 line of erosion, which may stop the supply of mechanical sediment 

 in any area. 



In the case of organic deposits, important bedding-planes indicate 

 the temporary disappearance of the rock-forming organisms from 

 the area ; and although this temporary disappearance may again be 

 brought about in many ways, climatic change is eminently qualified 

 to cause cessation from the deposit of organically-formed beds to 

 occur and recur with some approach to regularity. This was long- 

 ago recognized by James Croll, who suggested the formation of 

 important bedding-planes in some limestones owing to climatic 

 changes. He remarks l : — 



' In Derbyshire and in the South of Ireland and some other places, the 

 limestone is found in one mass of several hundred feet in thickness without 

 any beds of sandstoue or shale, but then it is nowhere found in one continuous 



1 ' Climate & Time ' 1st ed. (1875) p. 434. 



