THE CARBONIFEROUS PERIODS 475 



Summary of the Pennsylvanian 



Iron and Oil. — Beds of iron ore occur associated with coal. Such 

 beds are sometimes continuous, but the ore is often in the form of 

 nodules. The origin of these beds of iron ore is probably the same 

 as that of the " bog iron ore " which is accumulating in the swamps 

 and lakes of the present. Surface waters containing carbon dioxide 

 dissolved iron from the soil and rocks ; the dissolved mineral was then 

 carried to swamps and lakes by the streams, and there precipitated, 

 either as iron carbonate or iron hydroxide. 



Oil and gas occur in some of the sandstones of the Pennsylvanian 

 system in Illinois, Kansas, and Oklahoma. 



Duration. — The exact length of the Pennsylvanian Period is as 

 doubtful as that of the preceding periods, but is usually stated as 

 being about 2,000,000 years. In estimating the duration of former 

 periods it has been necessary to depend upon the rate of sedimenta- 

 tion, but in the Pennsylvanian an additional basis is afforded by the 

 coal. This measure is, however, inaccurate, since the rate of accu- 

 mulation is not definitely known. The aggregate thickness of the 

 coal in a single section of the Carboniferous is often 150 feet, and sec- 

 tions are known where the total thickness of the coal beds is 250 feet. 

 If the vigorous vegetation of a fertile region in North America to-day 

 were accumulated for 1000 years without loss and compressed to the 

 density of coal, it would form a layer only seven inches thick, but since 

 in the making of coal it is probable that four fifths of the vegetation 

 disappears as carbon dioxide (CO 2), methane (CH 4 ), and other gases, 

 the rate of accumulation would be only one and one half inches in 

 1000 years. It is readily seen, therefore, that 1,000,000 or 2,000,000 

 years may have been required for the accumulation of the Pennsyl- 

 vanian coal. 



Other Continents. 1 — The Pennsylvanian was the greatest coal- 

 producing period of the world. Workable beds occur in Great Britain 

 and Ireland and in all of the principal countries of Europe except 

 Norway, Sweden, Denmark, and Italy. The Pennsylvanian strata 

 in China, Asia Minor, and eastern Siberia contain coal beds, many 

 of which are of great value; in China, especially, coal beds of great 

 thickness and excellent quality have been reported. The Carbonif- 

 erous strata of Africa, Australia, and South America are seldom coal 

 bearing, but in some areas valuable deposits occur and much coal is 

 mined. 



