i.9 RHYTHM OF GEOLOGICAL CHANGES 19 



floods in North America. There were powerful earth movements at 

 the end of this period, at any rate in North America, known as the 

 Taconian revolution. The Silurian, lasting for 40 million years, 

 apparently included a single main cycle of inundation, ending in an 

 elevation of the land, which though slight in America, was marked 

 in Europe as the Caledonian revolution, producing the range of 

 mountains stretching across Scandinavia to Scotland and Ireland. 



GENERALIZED CONTINENTAL 



SUBMERGENCES 



PALEOZOI C 



ME S Z O I C 



CEN0ZO1C 



CAMBRIAN 0RO0VICIAN SIL DEVONIAN MISS PENN PERMIAN 





h-RiAssir iiiRA«if L0W U p PER 

 TRIASS'C JURASSIC CRET CRETACEOUS 



Tai n 1- Acadian Appalachian Nevadian Laramidt Cascadian 



Fig. 4. Diagrams of main changes of areas of land and water and in climatic conditions 



since the Cambrian. The chief times of mountain-building (orogenesis) in America are 



also shown. (Redrawn by permission from Textbook of General Zoology, 2nd ed. by 



W. C. Curtis and M. J. Guthrie, John Wiley & Sons, Inc., 1933.) 



Throughout these early Palaeozoic periods the fossils are entirely 

 those of aquatic animals, except for some traces of land plants and 

 arthropods at the end of the Silurian. The oldest remains of verte- 

 brates are fish-scales from the Ordovician (p. 125). Details of the 

 Palaeozoic climatic changes are not clear, but the fact that corals, 

 which can now live only in warm water, were alive over a considerable 

 part of the earth's surface suggests that conditions were warmer than 

 at present at least at some early Palaeozoic times. 



The Devonian is considered by some to include a single main 

 period, about 50 million years long, with one flood at the middle and 

 more arid conditions at the end, but other authorities divide it into 

 several periods. The first forests appeared at this time, and here, also, 

 are found tti3 first signs of vertebrate terrestrial life, in the form of 

 fossil lung-fishes and amphibians (p. 296). The period recognized as 

 Carboniferous in Europe includes two major periods of about 40 



