366 PALEOZOIC PERIODS 



underwent. The calculation has not been made for this country, 

 but for Great Britain Geikie states that the lower half of the 

 Palaeozoic group represents the waste of a plateau cut down to 

 base-level, larger than Spain and 5000 feet high. 



Very wide-spread disturbances of the earth's crust before the 

 beginning of the Palaeozoic era and at its close have produced 

 well-nigh universal unconformities with both the underlying pre- 

 Cambrian and the overlying Mesozoic rocks ; at only a few points 

 are transitional series found. 



Very early in Palaeozoic time were established the main geo- 

 graphical outlines which dominated the growth of the North 

 American continent, — a growth which was, for the most part, 

 steady and tranquil. These conditions may be briefly stated as 

 the formation of a great interior continental sea, divided from the 

 Atlantic and the Pacific by more or less extensive and variable 

 land areas. There are thus three principal regions of continental 

 development : those of the Atlantic and Pacific borders and the 

 interior. In addition, the eastern border is subdivided by pre- 

 Cambrian ridges into subordinate areas of deposition. At the 

 present time the surface rocks over the eastern half of the conti- 

 nent are prevailingly Palaeozoic, extending chiefly southward and 

 southeastward from the great pre-Cambrian mass of the north. 



Palaeozoic time was of vast length, probably exceeding that of 

 the combined Mesozoic and Cenozoic eras. 



The subdivisions of the Palaeozoic are very clearly marked, 

 locally often by unconformities, but on a wide scale by the 

 changes in the character of the fossils. There is some difference 

 in the practice concerning these divisions, not as to their limits 

 or order of succession, but merely as to their rank, whether cer- 

 tain ones should be called systems (periods) or series (epochs). 

 This is a difference more about names than facts. The succes- 

 sive steps of organic and geographical development are best dis- 

 played by dividing the group into six systems, or periods, which 

 are as follows, beginning with the oldest: 1. Cambrian; 2. Or- 

 dovician ; 3. Silurian; 4. Devonian; 5. Carboniferous; 6. Per- 

 mian. By many geologists the Ordovician and Silurian are 



