Frazer.] & J® [Dec. 4, 



from the bordering regions. The rocks in these latter regions, on the other 

 hand, are more and more magnesian, darker in color (usually greenish 

 or yellowish-green) and softer. They contain large quantities of chloritic 

 minerals, and are remarkable for the great number of white quartz dykes 

 which intersect them. 



These "arch-rocks" are very generally destitute of valuable minerals, 

 so far as they have been explored in York county, except on the fringe 

 of the South mountain, where they are in close proximity to a 

 series of iron ore deposits similar to and in fact continuous with those 

 known as the ores of the "Great," or "Cumberland Valley." But 

 though this juxtaposition would tempt one to connect these ores with the 

 rocks just spoken of, and though it is conceded that rocks of this age do 

 often carry iron ores, the strong probability is that the proximity is "acci- 

 dental," that is to say, that the ores occur at the foot of the mountain, 

 because having been originally imbedded (as constituents of minerals) in 

 the rocks which covered these slopes during the degradation and destruc- 

 tion of these latter they have been disintegrated, carried away from their 

 original place (sometimes not far off), and segregated in the soft and 

 unctuous clays to which these loose beds have been reduced. But it is not 

 improbable that some of these ores may have owed their origin to the 

 same kind of alteration taking place within the mass of the Huronian rocks 

 themselves. So that wherever the loose debris of higher formations (and 

 notably of the Hellam quartzite (Potsdam sandstone), which everywhere 

 abounds on the slope in boulders and blocks) will permit the undoubted 

 Huronian to appear near one of these great iron mines, it is likely to be 

 found that a part of the wealth of the latter consists in a somewhat pecu- 

 liar ore unlike the rest, which can be traced to its first resting place with- 

 in the bosom of the Huronian rocks. 



The belt of rocks which represents the Archaean in York county, lies, as it 

 may be said approximately, between two lines, one following Muddy 

 creek from its mouth in the Susquehanna to its ris;ht-angled bend, and 

 thence through Bryantsville to Constitution ; and the other commencing 

 opposite Turkey hill (in Lancaster county), and passing north-west of 

 Windsor post-office, south-east of Dallastown, and nearly through Glen 

 Rock post-office. The portion of the South mountain above referred to as 

 belonging to the same age is small in area within the county limits, and 

 occurring at one end of the chain of crystallophyllites where they appear 

 to sink beneath the newer limestones and shales ; its slopes are gentler ; it 

 has been subjected to greater erosion, and is covered for the most part with 

 the debris of more recent formations. This belt, thus defined, contains no 

 minerals which are yet mined (if we except the iron ores from the cate- 

 gory), but the soil formed by the chemical and mechanical action of the 

 atmosphere on its rocks is next in fertility to that of the limestone belt 

 itself. The rocks of the Archaean belt, thus defined, are intersected by but 

 few igneous dykes or trap, and this fact, taken in connection with the re- 

 markable prevalence of such dykes in the north-western part of the county, 



