METHODS — GEOLOGICAL 15 



notable changes in the system of living things. Such changes 

 in animals and plants may be compared to the almost imper- 

 ceptible movement of the hour-hand of a clock, while the re- 

 corded climatic revolutions and crustal movements often supply 

 the place of the minute-hand. It is obvious, however, that 

 if the hour-hand be wanting, the minute-hand alone can be of 

 very limited use. There have been a great many vast submer- 

 gences and emergences of land in the history of the earth, and 

 only the fossils can give us the assurance that we are comparing 

 the same movement in distant continents, and not two similar 

 movements separated by an enormous interval of time. 



It may thus fairly be admitted that it is possible to arrange 

 the rocks which compose the accessible parts of the earth's 

 crust in chronological order and to correlate in one system the 

 rocks of the various continents. The terms used for the more 

 important divisions of geological time are, in descending order 

 of magnitude, era, period, epoch, age or stage, and the general 

 scheme of the eras and periods, which is in almost uniform use 

 throughout the world, is given in the table, which is arranged 

 so as to give the succession graphically, with the most ancient 

 rocks at the bottom and the latest at the top. 



Cenozoic era 

 Mesozoic era 



[ Quaternary period 

 I Tertiary period 

 Cretaceous period 

 Jurassic period 

 Triassic period 

 Permian period 

 Carboniferous period 

 Devonian period 

 Silurian period 

 Ordovician period 

 Cambrian period 



^ _, , . [ Algonkian period 



Pre-Cambrian eras . , . j 



[ Archaean period 



Palaeozoic era 



