38 F\ REPORT OF PROGRESS. E. W. CLAYPOLE. 



\n>;[is again higher up in the Hamilton sandstone (No. 

 VIII g.)* 



As the eastern part of the sea seems to have sunken to 

 much greater depths than the western parts, the eastern 

 deposits in middle Pennsylvania were both coarser and 

 thicker than those in western Pennsylvania, Ohio, and the 

 western States, and contain a much larger proportion of 

 gravel beds, sandstones and shales to limestones, except at 

 the bottom of the whole series (No. II.) 



Towards the close of the palaeozoic age the bed of the 

 sea seems to have remained stationary and become filled 

 nearly to water level, producing continental swamps and 

 islands of carboniferous vegetation, resulting in the produc- 

 tion of the Coal Measures. A further subsidence however 

 allowed the deposit of an additional thousand feet of Per- 

 mian strata, before the great change came which elevated 

 the continent, drained away the sea waters to other regions 

 of the earth's surface, and established a new order of events. 



This change was brought about by a great pressure from 

 the direction of the present Atlantic seaboard, which thrust 

 the thick formations of Middle Pennsylvania sideways, 

 folding them together, elevating them into arches and de- 

 pressing them in troughs. 



As soon as this state of things commenced the frosts and 

 rains began to wear away the new continent, and a great 

 river system was established which began its work of trans- 

 ferring the waste of the land into the Atlantic ocean. This 

 river system is essentially that which is represented on our 

 maps. It has been at work through all ages since the close 

 of the coal era. The continent has never since then been 

 submerged, at least for any length of time sufficiently long- 

 to allow of the deposit of additional ocean sediments. 



The waste of the continent during the Triassic, Jurassic, 

 Cretaceous, and Tertiary ages has of course been enor- 



* Since this chapter was written, I have received the first volume of the 

 Geological Survey of Wisconsin. It may be merely a coincidence, but is 

 worth remark that a dry area existed over that State during the same period, 

 from the Lower TTelderberg to the Hamilton. There is consequent^' a pap 

 in the geological history of Wisconsin almost exactly coincident with that here 

 alluded to in Perry county. 



s 



