164 EXPLOITATION OF PLANTS 
realised in 1861, the boundary made between black coal 
and brown coal is often artificial and unscientific, and 
the distinction is far clearer in textbooks than it is in 
Nature. 
Important coals are to be found in each geological 
horizon in some part of the world; until we get right 
back to the Upper Devonian, which is the oldest epoch 
in which true coals occur. 
If the leading divisions of geological time are arranged 
vertically, a few of the more important coalfields of 
corresponding age may be indicated beside them to 
illustrate the above remarks. 
Geological Epoch. A few of the areas with commercially 
useful coal of corresponding age. 
Tertiary . . . . Germany, Canada, U.S.A., Japan, Spitz- 
bergen, New Zealand, etc. 
Upper Cretaceous . . France, Servia, Canada, U.S.A., etc. 
Lower Cretaceous. . Spain, Balkans, etc. 
Jurassic. . . . «. Sweden, Caucasus, Portugal, China, etc. 
Triassic. . . Virginia, U.S.A., Japan, Sweden, etc. 
Permo-Carboniferous . India, New South Wales, etc. 
Coal Measures. . . England, Scotland, France, Belgium, Ger- 
many, Russia, U.S.A., Canada, China, etc. 
L. Carboniferous . . Scotland, Russia, etc. 
Upper Devonian . . [Bear Island (Arctic) : true coal, but of more 
scientific than commercial value.] 
This indicates that each of these geological epochs is, 
in some part of the world, represented by local “* Coal 
Measures.” Now we know from the palzontological 
record, as well as from our theories of evolution, that the 
organic life of the epochs was ever changing, trending 
toward.genera and species liker and liker to those around 
us to-day. Hence, as coals are found throughout all 
these periods of time, we must either postulate the 
special survival of ‘‘ coal-forming” plants, or accept 
