12. pexx township. F 2 . 301 



The geological age of the {nip-dykes. 



Not one of these dykes is yet known to appear upon the 

 very summit of either Peters' or the Cove mountain. The 

 great Horse Shoe dyke ranges highest, running, as has been 

 shown, up to the topmost terrace of Peters' mountain on 

 its southern flank. This failure of the dykes to appear at 

 the summit proves the mountains to be older than the 

 dykes. 



Now since the Triassic red sandstone of York county is 

 cut by numerous similar dykes with which these Perry 

 county dykes seem to be connected, they must be not only 

 later than the coal measures, but of later age than the Trias ; 

 but as no such dykes are known in the Cretaceous beds of 

 the Atlantic seaboard, our dykes must be older than the 

 Cretaceous age. 



Nature of these Trap-rocJcs. 



The trap of Perry county is a hard, very tough, dark, 

 heavy and fine-grained dolerite containing grains of mag- 

 netic iron ore disseminated through the mass, readily dis- 

 coverable by crushing a small piece with the hammer and ap- 

 plying a magnet when the magnetite immediately clings to 

 it. The presence of this material is partly the cause of the 

 decay which takes place at the surface of the trap. Under 

 the action of moisture the magnetite becomes rusty and 

 passes into brown hematite. The outer layer of stone is 

 softened and changes color to a rusty yellow. This outside 

 layer scales off and the process is repeated upon the new 

 surface thus exposed. In this way from year to year a red 

 clay soil is produced, by the disintegration of the other.mate- 

 rials of the rock, felspar and hornblende, colored by the iron 

 oxide. In consequence of the abundance of this red clay 

 along the course of the trap it is usually called by the resi- 

 dents of the neighborhood "iron ore," or "magnetic ore 

 rock." But it is not likely that any merchantable iron ore 

 will be found along the lines of these dykes. 



