THE PENNSYLVANIAN PERIOD 



387 



tropical jungles. Thick bark is another feature shared by the 

 coal trees and those of our cooler countries to-day. It seems 

 not improbable, therefore, that the climate under which 

 many of the coal marshes nourished was more like that of 

 Canada than of Florida or the Amazon. 



Great length of the period. While it is not possible to 

 calculate exactly the duration of geologic periods, some 

 rough estimates made for the Pennsylvanian are of interest. 

 For the growth of the vegetation which made the coal seams 

 in a single locality 1,000,000 to 2,500,000 years would seem 

 to be required, and, for the sediments in which they are inter- 

 bedded, at least as much more. It is therefore possible that 

 5,000,000 years were included in this single period. 



QUESTIONS 



1. In Pennsylvania there are thick beds of conglomerate and 

 sandstone which contain no fossil shells, but an abundance of plant 

 leaves in certain layers. Can you suggest the origin of these rocks ? 



2. The sandstone which 

 lies at the base of the Coal 

 Measures in many parts of 

 this country and Europe is 

 sometimes called the mill- 

 stone grit. Can you suggest 

 why this name was given it ? 



3. The accompanying cross section (Fig. 410) shows a narrow, 

 winding bed of sandstone lying in a coal seam. 



Such things are known to the miners as "cut 

 outs." How may such a feature have been 

 produced ? 



4. Tell all the events and changes which 

 you find recorded by the accompanying sec- 

 tion taken from the Coal Measures of Ohio 

 (Fig. 411). 



5. Coal seams are often broken by faults. 

 If in following a certain coal seam in a mine, 

 you should encounter such a fault, how could 

 you tell whether to hunt for the lost continua- 

 tion of the bed at a higher or lower level ? 



6. The Pennsylvanian strata are the latest widespread Paleozoic 



FIG. 410. A sandstone "cut out" in a 

 coal seam. 



FIG. 411. Section 

 from Ohio Coal 

 Measures. 



