DIVISIONS OF GEOLOGICAL TIME 377 



DIVISIONS OF GEOLOGICAL TIME. 



The student of Palaeontology constantly requires to 

 refer to the names which have been given to the divisions, 

 great and small, into which the fossiliferous rocks and 

 the time of their deposition have alike been divided. 

 These are here given in tabular form for convenience of 

 reference. For the grounds on which their limits are 

 fixed the reader is referred to textbooks of Stratigraphy, 

 or he will find a reasoned statement in a paper by 

 MM. Munier-Chalmas and de Lapparent : "Note sur la 

 nomenclature des terrains sedimentaires," Bull. Soc. Geol. 

 France, 3 e ser., t. xxi. (1893), P- 43$- 



Some possible sources of confusion must be pointed 

 out. First, the necessity for a double series of terms 

 more or less local stratigraphicaRerms (such as Wenlock 

 Limestone, Carboniferous Limestone, Gault) and terms 

 of world-wide, or at least international, application (such 

 as Cambrian, Visean, Cenornanian). Both have their 

 own usefulness, but the attempt to use terms of the first 

 category in place of those of the second has led to much 

 confusion in correlation ; while forgetfulness of the fact 

 that the second category should apply, as far as possible, 

 over the whole world has led geologists to coin some 

 unnecessary synonyms. The limits of the first set of terms 

 are determined by marked changes in the character of the 

 sediments laid down in a limited area; those of the 

 second should be determined primarily by widespread 

 changes of fauna or flora, and secondly by great changes 

 in the distribution of land and sea (the biological and 

 geographical changes being intimately related). 



The tables on pp. 378-380 must not be taken as fixed 

 and settled. Geologists are not in complete agreement 

 as to the relative importance of many divisions : what is 

 a series to one is only a stage to another, and nomencla- 

 ture varies accordingly. A few synonyms, etc., are given 

 here, in descending order : 



Montian is included in Danian by some. 

 Aturian = Maestrichtian + Campanian . 

 Senonian = Aturian + Emscherian. 



