346 PALAEOZOIC SYSTEM OF KOCKS. 



Coal-measures mainly of fresh-water formation — the former mainly of 

 limestone, the latter mainly of sands and clays ; the fossils of the for- 

 mer are, therefore, mainly marine animals, of the latter mainly fresh- 

 water and land animals and plants, though marine animals are also 

 found. In both Europe and America the coal-basins consisting of the 

 latter are underlaid by the former, which, moreover, outcrop all around, 

 forming a penumbral margin to the dark areas representing coal-basins 

 on geological maps (see map, page 291). Between these two, or, rather, 

 forming the lowest member of the Coal-measures, there is, in many 

 places, a thick, coarse sandstone, called the millstone grit. 



After this general contrast, we will now concentrate nearly our 

 whole attention upon the Carboniferous period proper ; because in this 

 middle period culminated all the more striking characteristics of the 

 age. In speaking of the life-system, however, we will draw from both 

 sub- Carboniferous and Carboniferous indifferently. The Permian we 

 shall treat only as a transition to the next era. 



Carboniferous Proper — Rock- System or Coal-Measures. 



The Name. — The Carboniferous period is but one of the three peri- 

 ods of this age. The Carboniferous age is, again, but one of the three 

 ages of the Palaeozoic era, while the Palaeozoic era is itself but one of 

 the four great eras, exclusive of the present, of the whole recorded his- 

 tory of the earth. The Carboniferous period, therefore, is probably not 

 more than one thirtieth part of that recorded history. Yet, during that 

 period were accumulated, and in the strata of that period (Coal-meas- 

 ures) are still inclosed, at least nine tenths of all the ivorked coal, and 

 probably nearly nine tenths of all the workable coal in the tvorld. It is 

 essentially the coal-bearing period. When we remember that every 

 geological period has its characteristic fossils, by means of which the 

 formation may be at once recognized by the experienced eye, it is easy 

 to see the importance of this simple fact as a guide to the prospector. 

 It has been estimated that the money, time, and energy, uselessly ex- 

 pended in the State of New York in explorations for coal, where any 

 geologist might be sure there was no coal, would suffice to make a com- 

 plete geological survey of the State several times over ! The same is 

 true of Great Britain and many other countries. 



Thickness of Strata. — Although constituting so small a portion of 

 the whole stratified crust of the earth, the coal-measures are in some 

 places of enormous thickness. In Nova Scotia they are 13,000 feet ; in 

 South Wales, 12,000 feet ; in Pennsylvania, 4,000 feet ; in West Vir- 

 ginia, over 4,500 feet ; in Indian Territory, 8,000 to 10,000 feet.* 



Mode of Occurrence of Coal. — Such being the thickness of the coal- 



* American Geologist, vol. vi, p. 238. 1890. 



