GENERAL OBSERVATIONS. 393 



parts to seven miles or more, while parallel with it in the Interior 

 basin the average was comparatively small. The review of the limits 

 of the successive formations, on p. 379, shows that even the minor 

 changes took place under the influence of oscillations having this 

 general course. 



The Lower Silurian uplift from Cincinnati to Tennessee conforms 

 to this system. In accordance also with it, the Coal measures in 

 Pennsylvania, to the top of the Pittsburg series, were elevated so 

 that their marshes became dry before the higher beds were laid 

 down; and these upper beds, with the whole region west to the 

 Mississippi, before the Permian (p. 371). 



The Appalachian region, including from the Gulf of St. Lawrence 

 to Alabama, lies parallel with one great branch of the Azoic dry 

 land, C C, on map, p. 136, and also with the Atlantic Ocean. The 

 Appalachian oscillations therefore conformed in direction with one 

 of the two Azoic systems (p. 147) : they were but a continuation 

 of the series that prevailed while the Azoic age was in progress. 



With regard to the region west of the Azoic, our information is 

 yet scanty: sufficient, however, is known to make it apparent that 

 the increase of dry land was from the Azoic to the southwest, or 

 corresponding to oscillations parallel to the Rocky Mountains. 

 The direct effect of such oscillations is manifest in the Illinois 

 uplifts preceding the Coal measures, for they are parallel to the 

 Rocky Mountain chain and the Pacific coast-line. This, then, 

 was a second grand direction of oscillations. It was parallel with 

 the northwestern branch of the Azoic, B B, on map, p. 136, and 

 corresponded to the second of the two series that prevailed during 

 the Azoic age. 



It is hence apparent that, whatever the forces at work in Azoic 

 time, they continued to act in the same general direction through- 

 out the Palaeozoic. The action of the two systems of forces to- 

 gether evidently produced the great amount of subsidence adjoin- 

 ing the Canada Azoic, where the thick deposits of the Huronian 

 and Potsdam periods were formed, and where finally the basins of 

 the great lakes were made. These, and nearly all the lakes of North 

 America, lie near the limit between the oscillating part of the con- 

 tinent and the stable Azoic. 



5. Cotemporaneous movements in the American and European continents. — 

 The fact that the continent of Europe was above the ocean, and 

 in that condition which was characteristic of the Coal period, at 

 the same time with North America, shows a cotemporaneousness 

 in the oscillations of the crust on the opposite sides of the Atlantic 

 Ocean. This concordance will be better apprehended when it is 



