80 F*. REPORT OF PROGRE88. E. W. OLAYPOLE. 



between the other rocks, filling up cracks and cavities and 

 baking and hardening by its heat the strata through which 

 it flowed. When cooled the fluid matter became hard, and 

 is now known as intrusive or trap-rock. 



These trap-rocks of Perry county are but a part of a great 

 system of dykes which traverse the New Red Sandstone 

 rocks of York and Adams counties. 



Full details of these trap dykes are given in the reports 

 on Penn and Rye townships, so that it is unnecessary here 

 to do more than call attention to them. One of them ap- 

 pears to cross the Susquehanna, a mass of trap appearing 

 opposite Duncannon in the cutting on the Northern Cen- 

 tral railway. 



