298 HISTORICAL GEOLOGY. 



The fossils of the former are, therefore, marine animals ; 

 those of the latter mainly land-plants, and fresh-water and 

 land animals. In both Europe and America the sub- 

 carboniferous underlies the coal-measures and outcrops 

 around, and thus forms a penumbral margin about the 

 black areas representing coal-fields on geological maps 

 (see Fig. 169). 



After this brief comparison and contrast, we shall now 

 concentrate our attention on the coal-measures, because 

 all the characteristics of the Carboniferous age culminate 

 there. In speaking of the fauna, however, we shall take 

 the two together. The Permian will be treated as a 

 transition to the Mesozoic. 



Carboniferous Proper Rock-system, or Coal- Measures. 



Name. The Carboniferous period is but one of three 

 periods of the Carboniferous age. The Carboniferous age 

 is but one of the three ages of the Paleozoic era. The 

 Paleozoic era is but one of the four great eras, exclusive 

 of the present. The Carboniferous period, therefore, is 

 but a small fraction, certainly not more than one twentieth 

 to one thirtieth of the recorded history of the earth. Yet, 

 during this period were accumulated, and in its strata 

 were preserved, and are now found, nine tenths of all 

 the coal used by man. The name carboniferous, for the 

 period, and coal-measures, for the strata, is surely, there- 

 fore, appropriate. 



Thickness of the Strata. Although so small a por- 

 tion of- the whole strata of the earth, these coal-measures 

 are often, locally, of great thickness. In Nova Scotia the 

 coal-measures, exclusive of the sub-carboniferous, are 

 14,000 feet thick, in Wales 12,000, in Arkansas ^,000 

 (Branner), and in West Virginia 5,00(1. 



Mode of Occurrence of Coal. Such being the thick- 

 ness, it is evident that but a small portion is coal. \\\ 



