322 



HISTORICAL GEOLOGY. 





from the Archaean nucleus. 2. There was also a pro- 

 gressive change in the constitution of the atmosphere, 

 especially by removal of excess of water and C0 2 , fitting 

 it for the introduction of higher animals. 3. In connec- 

 tion with these physical changes, there were also progres- 

 sive changes in life-forms.* 

 Appalachian Kevolu- 

 tion. Thus we see a slow, 

 steady, progressive change 

 during the era. But now, 

 at the end, there occurred 

 one of those great and rapid 

 changes in physical geogra- 

 phy and climate which mark 

 the end of the eras, and 

 a corresponding sweeping 

 change in the forms of life. 

 The Appalachian chain was 

 formed at this time, and is 

 its monument, and therefore, 

 by American geologists, it is fitly called the Appalachian 

 Revolution. The place of the Appalachian chain during 

 the Silurian and Devonian eras was the marginal sea- 

 bottom of the great interior Paleozoic Sea, receiving 

 sediments until 30,000 feet were accumulated. During 

 the Carboniferous it was sometimes an inland sea-bottom, 

 sometimes a coal-marsh, and sometimes, perhaps, a lake, 

 but always receiving sediment until 10,000 more feet were 

 accumulated. Now, at last, it yielded to the ever-increas- 

 ing lateral pressure, and was folded and crumbled with 

 all its coal-beds, and swelled up into a great mountain- 

 range. It has since been sculptured by erosion into its 

 present forms. 



We have said that the change of life-forms produced 



* For a fuller account of this important point, the teacher is re- 

 ferred to the larger work. 



FIG. 256. Fossil rain -prints of the Coal 

 period. 



