Handbook of Paleontology 213 



Ordovician of authors. These periods are not of equal 

 length, nor have they been of equal importance in the 

 development of life. They can be recognized in any part 

 of the world by their characteristic fauna, and it is upon 

 these faunas that their separation is ultimately based, 

 though locally the systems of rocks may often be clearly 

 marked by unconformities. As shown in the table above, 

 in the older Paleozoic series of rocks no vertebrate remains 

 are found at all. They are found for the first time in the 

 Silurian, and even those are doubtful vertebrates. 

 Invertebrates are just as abundant in the younger 

 as in the older Paleozoic even though characterized by 

 the higher forms of life. 



The rocks of the Paleozoic are conglomerates, sand- 

 stones, shales, and limestones and dolomites with quite 

 extensive areas of metamorphic rocks and associated 

 igneous masses, but on the whole Paleozoic strata are far 

 less altered and deformed than those of the preceding 

 eras. In Europe the estimated total thickness of Paleo- 

 zoic sediments is about 100,000 feet, though of course not 

 in any one place. In North America the greatest thick- 

 ness is exposed in the much folded and profoundly de- 

 nuded Appalachian Mountains area. The maximum 

 thickness of marine deposits of all kinds in our continent 

 is approximately 75,000 feet, which has been estimated 

 as equivalent in time of depositions to 43,000 feet of lime- 

 stone. North America is wonderfully rich, particularly 

 in the eastern half of the United States and Canada, in 

 the long succession of Paleozoic formations with abun- 

 dant fossils; and here in contrast to Europe, the highly 

 fossiliferous strata west of the Appalachian area in the 

 Mississippi valley and in southern and medial Canada 

 occur in almost the same position in which they were 



