THE MAKING OF THE GEOLOGICAL TIME-SCALE. 31 



tions. In geological text-books and in other geological liter- 

 ature of America, when Silurian, Jurassic, or Cretaceous is 

 used alone, the Silurian, Jurassic, or Cretaceous Period is 

 meant. In English literature, however, system is generally 

 understood when not otherwise specified. 



Geological Systems the Standard Units of the Time-scale. The 

 final result of these attempts to arrange chronologically the 

 geological formations is found in the standard classification of 

 the systems. The systems were originally groups of success- 

 ive rock-formations; their limitation was therefore deter- 

 mined, in the first place, by the extent of the rocks in the 

 particular region where they were first defined. Hence the 

 series of formations constituting an original system is in each 

 case a standard of reference, and its general application is 

 accomplished by determining its equivalent formations in 

 other regions. 



The time-periods are the periods represented by these 

 systems; hence the periods of time-duration receive the 

 names of the systems which were formed during the periods. 

 The expression, the Cambrian Period, means the period of 

 time during which the Cambrian system of rocks was forming, 

 or the period in which the Cambrian faunas and floras lived. 

 It is all-important to know what formations make up these 

 standard systems ; for only as other rocks contain the same 

 faunas or floras can they be identified as of equivalent age, 

 and therefore as belonging to the same system. The real 

 time-indicators are, therefore, the fossils, although the rock- 

 formations which held the fossils give us the names for the 

 chief divisions of the time-scale. 



THE GEOLOGICAL SYSTEMS. 



Cambrian System. The CAMBRIAN SYSTEM was defined by 

 Sedgwick, and the name was applied to formations studied in 

 North Wales. In the original definition of the system (1835), 

 in a paper by Sedgwick and Murchison,* the extension of the 



* " On the Silurian and Cambrian Systems, exhibiting the order in which 

 the older sedimentary strata succeed each other in England and Wales." 

 British Assoc., August, 1835. 



