1885.J "«-'5 [Frazer. 



the farther one follows the direction of the arch to the south-west. I have 

 elsewhere given reasons for the hypothesis that this anticlinal joins 

 and continues the anticlinal of the Buck Ridge * near Conshohocken, a 

 few miles north-west of Philadelphia, on the Schuylkill river, traversing 

 Lancaster and Chester counties, a little south of the Chester valley. But 

 at Conshohocken, the anticlinal is represented by Laurentian gneiss, 

 while in Lancaster and York counties, the Huronian schists, which have 

 been torn off by atmospheric denudation at the former locality, still re- 

 main ; and still farther to the south-west it is not unlikely that.even more 

 recent sheathings may be found, unless the axis be broken or bent, and 

 rise also in this direction. The main fact, which it is my purpose to 

 emphasize here, is that the same structure of arch evidently affects an 

 enormous thickness of beds ; in all probability is traced in the flexed rock 

 masses of at least two entirely different geological periods, and may pos- 

 sibly be discovered in those of yet others outside of the limits of the field 

 which it is my purpose to describe. 



A somewhat arbitrary division has been made by the writer between 

 the rocks of the Huronian and those of the next following age. The line 

 which constitutes this division may be seen passing through the southern 

 part of Lower Windsor, the middle of Windsor, the eastern part of Spring- 

 field, including Codorus, and reaching the Maryland line a short distance 

 east of the boundary dividing Manheim from West Manheim township. 

 This line does not profess to be, and in all probability is not an accurate 

 line of demarcation between the two formations. It was adopted as an 

 approximate dividing line between two regions which exhibit lithological 

 characteristics diverging from each other in a degree proportional to the 

 distance on either side of it. The same is true of the line which separates 

 these lower rocks from the triangular area in the extreme southeastern 

 corner of the county, in which are found the famous Peach-Bottom roof- 

 ing slates. These two lines, which are in the average parallel to each 

 other, are approximate boundaries only between the two regions, and that 

 filled by the rocks of the McCall's Ferry or Tocquan Creek anticlinal. 

 The rocks of the latter belt are strongly marked crystallteed rocks, f i. e., 

 their structure is coarse, and the minerals which compose them are large 

 and well crystallized, especially along the central parts of the belt. The 

 rocks of the two bordering regions just mentioned are more crystalline, 

 i. e., crystallized imperfectly or in much smaller masses, besides having 

 other differences in kind. For example, the arch-belt (if I may be per- 

 mitted to express it so), contains larger amounts and larger specimens of 

 Muscovite, and more potash micas generally. The rocks are lighter, and 

 not infrequently enough feldspar is found to give them a decidedly gneissic 

 character; and the more so in general terms, the farther one gets away 



*See "Theses presentees a la Faculte des Sciences de Lille. University de 

 France," &c, 1882, and "History of Lancaster County," &c, Phila., Everst & 

 PeckPubl., 1883, p. 3. 



fSee note at the end. 



