CHAPTER XXII 



THE PALEOZOIC PERIODS — CAMBRIAN 



The Palaeozoic is the oldest of the three main groups into 

 which the normal fossiliferous strata are divided ; it forms the 

 first legible volume of the earth's history, and in interpreting it 

 speculation and hypothesis play a much less prominent part than 

 in the pre- Cambrian volume. The Palaeozoic rocks are con- 

 glomerates, sandstones, shales, and limestones, with quite exten- 

 sive areas of metamorphic rocks, and associated igneous masses, 

 both volcanic and plutonic. The thickness of these rocks is very 

 great, estimated in Europe at a maximum of 100,000 feet. This 

 does not imply that such a thickness is found in any one place, 

 but that if the maximum thicknesses of each of the subordinate 

 divisions be added together, they will amount to that sum. In 

 this country more than 30,000 feet of Palaeozoic strata are ex- 

 posed in the much folded and profoundly denuded Appalachian 

 Mountains, but in the Mississippi valley they attain only a fraction 

 of that thickness. These rocks are, in the vast majority of cases, 

 of marine origin, but some fresh-water beds are known, and 

 very extensive swamp deposits have preserved a record of much 

 of the land life of the era, especially of its later portions. That 

 there must have been land surfaces is abundantly shown by the 

 immense thickness and extent of the strata, all of which were 

 derived from the waste of the land. Both in Europe and in North 

 America the land areas were prevailingly toward the north and are 

 doubtless indicated, in part, by the great regions of the pre-Cam- 

 brian metamorphic rocks. The general character of the Palaeo- 

 zoic beds shows that they were, in large measure, laid down in 

 shallow water in the neighbourhood of land. Their great thickness 

 indicates, further, the enormous denudation which the land areas 



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