CHAPTER XIII 

 SUMMARY OF PALEOZOIC HISTORY 



"We have defined geology as the history of the evolution of 

 the earth. Evolution, therefore, is the central idea of geology. 

 It is this idea alone which makes geology a distinct science. This 

 is the coherent principle which unites and gives significance to all 

 the scattered facts of geology — which cements what would other- 

 wise be a mere incoherent pile of rubbish into a solid and symmetri- 

 cal edifice. It seems appropriate, therefore, that at the end of the 

 long and eventful Paleozoic era we should glance backward and 

 briefly recapitulate the evidences of progressive change (evolu- 

 tion)." 1 



Paleozoic Rocks 



Paleozoic rocks are dominantly sandstones, conglomerates, 

 shales, and limestones of typical, marine, sedimentary character, 

 though continental deposits also are common, such as fresh-water, 

 swamp, or lagoon deposits of the Pennsylvanian in the eastern 

 Mississippi Basin and the "Red Beds" formed in great salt lakes 

 of Permian age in the southwestern United States. 



The marine strata furnish abundant evidence, by the presence 

 of ripple and wave-marks, the coarseness of the clastic materials 

 (conglomerates and sandstones), etc., that they were deposited in 

 shallow (epicontinental) seas, and never in really deep ocean 

 water. Continental deposits are also abundantly represented. 



In Europe the estimated maximum thickness of Paleozoic strata 

 is 75,000 to 100,000 feet. It must be remembered, however, that 

 this does not mean that such a great thickness of strata is present 

 in any one locality, but rather that this represents the sum-total 

 of the greatest thicknesses of the different formations of the con- 

 tinent. 



A thickness of more than 25,000 feet of strata (largely clastic) 

 actually piled layer upon layer may now be seen exposed in the 



1 J. LeConte: Elements of Geology, 5th Ed., p. 421. 

 194 



