466 HISTORY OP 



Next above this lies "formation number 11" of the state- 

 survey, including the tracts of limestone found m our valleys 

 and level districts, and approaching the base of most of the 

 larger hills, but seldom itself rising into ridges much above the 

 general undulations of the surface. 



The northern border of the county is made up of a formation 

 of red and grey shales, or soft slates, grits, and pudding stones, 

 furnishing in some places a material sufficiently hard and 

 compact to afford an excellent material for mill stones. These 

 are accordingly wrought out of the large detached fragments 

 found upon the surface in Cocalico township. In several loca- 

 lities the same formation has afforded indications of coal, but 

 as it is entirely distinct from the great coal formation of the 

 commonwealth, it is very probable that veins which can be 

 advantageously worked, will never be discovered. Iron ore is 

 of rather frequent occurrence, and we have met with indica- 

 tions (>f copper. In several localities on the southern border 

 of this series, the curious calcareous rock called Potomac 

 marble occurs; namely, east of Bainbridge, and north of Man- 

 heim. 



But the most interesting feature in the Conewago hills, 

 is the large amount of weathered blocks upon the surface, of a 

 bard grey stone made up of white and black particles. This is a 

 trap rock of the variety called greenstone, and identical in 

 composition with the smaller ridges which traverse different 

 parts of the country, under the name of ironstone, a mineral 

 remarkable for the sonorous ring produced when struck. The 

 finer texture of the latter is produced by the rapid cooling of 

 the material, consequent upon the comparatively small quan- 

 tity of matter; this rock being of igneous origin, and injected 

 from below in a melted condition. In the Conewago rock 

 the quantity of material is so great, that in the length of time 

 required to solidify, the constituents were able to enter into 

 combinations ; or crystallize, in a manner; whence the felspar 

 and hornblend appear in distinct particles. 



A flood sweeping across these hills, has carried large blocks 

 of the rock for miles southward, and beyond the reach of the 

 highest floods of the Susquehanna. That a powerful current 

 swept over the .country from the north-west, is proved by the 

 additional fact that primary sienitic pebbles are found among 



