Frazer.] 6.)4: [Dec. 4> 



The Archaean Rocks of Youk County.* 

 Crystalline Schists (a 2 ). 



I have not seen in York county any rocks which I considered to be of 

 Laurentian age. If there be any, they are to be sought in the portion of 

 the South mountain, which is included in parts of Carroll and Franklin 

 townships, but it is very improbable that any will be found there. The 

 same may be said of the Norian, which is simply another name for what 

 was once called "Upper Laurentian." There remain then only the 

 Huronian, the Mont Alban and the Taconian, for the Keweenian is not 

 known in this part of the United States. The lowest member of the 

 Archaean series, which has been recognized in York county is the Huron- 

 ian, and if I be not in error, the rocks of this age form the greater part, if 

 not all, of its lower strata. On the accompanying geological map it is col- 

 ored a pink of medium tint, and lettered "a 2 ," as well as all that pre- 

 viously referred to in Carroll and Franklin townships forming the South 

 mountain. 



Crossing the Susquehanna somewhat obliquely a broad fiat arch of 

 these rocks becomes evident in plotting the observations on section lines 

 along either the right or left bank of the river. f 



The perpendicular thickness of the Huronian rocks which constitute 

 the visible parts of this arch has been calculated by me to amount to 

 fourteen thousand four hundred feet, or 2.7 miles (or 4.3 kilometers), 

 measuring from the lowest rocks exposed a short distance above McCall's 

 Ferry to the base of the Peach-Bottom slates. This arch (or anticlinal) 

 is a very important feature in the geology of this part of the State ; for 

 it is not improbable that it is the leading element in the structure of a 

 broad belt of rocks extending from a point at least north of the Schuyl- 

 kill river (and not improbably even within the New England States) to 

 and into the State of Alabama. 



But whether this carefully considered hypothesis be true or not, there is 

 not the slightest reason for doubting that the rocks of this part of the 

 county form the floor on which all the others in the county were laid 

 down. Another fact in relation to this flat arch or anticlinal remains 

 to be considered, viz : the line along its crown (or along the top of the 

 arch) appears not to have been an horizontal line after the last great earth- 

 crust movements, of which we can find evidence in this part of the con- 

 tinent, had been completed ; the axis of this arch appears to have sloped 

 upwards, from the west of south to the east of north ; and to say 

 that this axis rises towards the north-east, is to say that, judged from 

 our present surface, the lower (and consequently older) beds of this 

 arch rise nearer to that surface, the farther one follows this direction of 

 north-east ; and of course these same rocks sink lower beneath the surface 



*See Note 7 at the end. 



tSee these sections by the author in atlas accompanying volume CCC, 2d 

 Geological Survey of Pennsylvania. 



