354 FOSSILS 



Such an extreme case as the fossil plants of Greenland is sufficient 

 evidence without further corroboration. 



III. Classification of Geological Time 



The method of making the divisions and subdivisions of geo- 

 logical time is not yet a fixed one, and there is much difference in 

 the usage of various writers. The names of the divisions also 

 have been given at various times and in many lands, according to 

 no particular system. Most of these names have been taken from 

 the locality or district where the rocks in question were first 

 studied ; as Devonian from Devonshire, Jurassic from the Jura 

 Mountains. Some are named from a characteristic or prevalent 

 kind of rock, such as Cretaceous (Latin err fa, chalk) and Carbonif- 

 erous. Of late there has been a tendency toward a more uniform 

 method of nomenclature, and to the use of one set of terms for 

 the divisions of time, and another and corresponding set for the 

 divisions of the strata. The grander divisions of time are called 

 eras, and in descending order we have periods, epochs, and ages. 

 The following table represents the divisions in the scale of time 

 and the scale of rocks which have been adopted by the Inter- 

 national Geological Congress. 



Time Scale Rock Scale 



Era Group 



Period System 



Epoch Series 



Age Stage 



Substage 



Zone 



It will be observed that the subdivision is carried further in the 

 scale of rocks than in that of time, because of the generally local 

 character of these minor subdivisions. The names employed are, 

 as yet, the same for both scales, and we speak of the Palaeozoic 

 Era or Group, and of the Silurian Period or System. It has been 

 proposed to give separate names to the divisions of the two scales, 



