THE PENNSYLVANIAN PERIOD 169 



Europe, and northern China contain the most important coal 

 fields. The map (Fig. 93) gives a general idea of the locations of 

 the coal fields of eastern North America, though several of the 

 areas of coal-bearing Pennsylvanian rocks are really somewhat 

 larger than this surface distribution (or outcrop) map shows. 

 These areas, largely underlain with workable coal, are as follows: 



(1) Anthracite field of eastern Pennsylvania — 484 square miles; 



(2) Appalachian field from western Pennsylvania to Alabama — 

 70,000 square miles; (3) Eastern Interior field in Indiana, Illi- 

 nois, and Kentucky — 50,000 square miles; (4) Northern In- 

 terior field in Michigan — 11,000 square miles; (5) Western 

 Interior field from Iowa to Oklahoma — 72,000 square miles; 

 (6) Texas field — 13,000 square miles; and (7) Nova Scotia- 

 New Brunswick field — 18,000 square miles. Thus in eastern 

 North America a total of about 235,000 square miles is mostly 

 underlain with workable coal of Pennsylvanian age. Consider- 

 able coal of this age also occurs in Alaska. 



Iron ores of some importance are found in the carbonate and 

 oxide forms as bedded deposits in Pennsylvanian rocks. Such 

 deposits were formed by precipitation, in the marshes and swamps, 

 of the iron brought down from the lands in soluble form. The 

 principal deposits occur in western Pennsylvania, eastern Ohio, 

 and northern West Virginia. 



Pennsylvanian rocks also yield considerable oil and gas espe- 

 cially in Illinois, Kansas, and Oklahoma. 



Life of the Pennsylvanian 



In all of the preceding periods, our studies of organisms 

 have been chiefly confined to marine forms because either 

 they only existed, or predominated, or because they have left 

 us the most abundant records. Rocks of the Pennsylvanian sys- 

 tem are the earliest to carry abundant records of land plants 

 and animals (Amphibians), and for the first time our principal 

 discussion of the life of a period will deal with such forms. The 

 Coal Measures and their enclosed organic remains have been 

 studied in unusual detail because of the economic value of the coal. 



Plants. — The plant life of Pennsylvanian time was very 

 prolific and the records for this period are far more abundant than 

 for any other Paleozoic period, one reason for this unusually full 



