GEOLOGICAL HISTOBY <>F THE COUNTY. F 2 . 37 



Carlisle and Chambersburg. No. II is more than 6000 feet 

 thick in Blair county, but may not be 3000 feet thick at 

 Harrisburg and Carlisle. No. Ill is more than 5000 feet 

 thick on the Lehigh and Delaware rivers, but less than 1000 

 feet thick in Bedford county. And so with the other for- 

 mations. 



It appears then that more than six miles of material ac- 

 cumulated in middle Pennsylvania while it was the bed of 

 a sea ; so that in places where these rocks exist in full thick- 

 ness a bore-hole would have to be sunk to that depth to 

 reach the azoic rocks on which they lie. 



Changes in the depth of the sea seem to have taken place 

 at certain stages in the long history of the palaeozoic de- 

 posits. Either different parts of the sea filled up with un- 

 equal rapidity ; or its bed settled unequally ; or slight up- 

 ward movements of the crust of the earth took place along 

 certain lines or at certain points, so as to expose parts of the 

 sea bottom to the air for short times, during which there 

 could be no deposits at such places. But in spite of these 

 irregularities, it is evident that there was an almost contin- 

 uous subsidence of the crust of the earth beneath the palae- 

 ozoic sea from first to last ; for, otherwise there could not 

 have been a series of formations six miles thick deposited. 

 The interruptions in the process were local, and resulted in 

 the loss of only a few hundred feet of rock measures. At 

 such places there must have been islands or sand-banks in 

 the sea, like the one at Marysville, described in the report 

 on Rye township ; and another may be indicated by the 

 Bridgeport sandstone in the western part of the county. 



By whatever cause such changes in the condition of the 

 palaeozoic column may have been produced, they seem to 

 have had an important effect upon the animal life which 

 prevailed in the sea in different ages, changing their condi- 

 tions of existence, causing some species to perish or migrate 

 to distant regions, and bringing in others to occupy their 

 places. In some cases it seems to have happened that 

 species driven away for a time afterwards returned. The 

 genus JRensselceria, for example, disappears from the rocks 

 at the top of the Oriskany sandstone (No. VII,) but ap- 



