GENERAL OBSERVATIONS. 393 



valley and New England, there were dislocations of the strata and 

 extensive uplifting at the close of the Devonian, making high ridges, 

 but no true mountain range. But, in general, over the Continental 

 Interior, and along the Appalachian region south of New York, the 

 strata from the bottom of the Silurian to the top of the Carboniferous 

 make an unbroken series, with no unconformability except the slight 

 want of parallelism by overlap, which the great oscillations at times 

 occasioned (p. 305). The extent of the series, and the vast length of 

 time occupied by those passing ages, make tin's exemption from great 

 disturbances a subject of profound importance in American geological 

 history. 



4. Direction of Oscillations. — The direction of the oscillations of 

 the continent may be learned from the course of the region along 

 which, through the successive periods, the greatest amount of change 

 of level took place. One such region is the Appalachian, in which the 

 subsidence, as has been shown, amounted in some 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 for- 

 mations, on p. 389, shows that even the minor changes took place 

 under the influence of oscillations having this general course. 



The Lower Silurian uplift, from Lake Erie to central 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. 368). 



The Appalachian region lies parallel with one great branch of the 

 Archaean dry land, C C, on map, p. 149, and also with the Atlantic 

 Ocean. The Appalachian oscillations therefore conformed in direction 

 with one of the two Archaean systems (p. 160) : they were but a con- 

 tinuation of the series that prevailed while the Archaean age was in 

 progress. 



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

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

 increase of dry land was from the Archaean to the southwest, or cor- 

 responding 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 

 Archaean, B B, on map, p. 149, and corresponded to the second of the 

 two series that prevailed during the Archaean age. 



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



