APPALACHIAN FOLDS OF CENTRAL PENNSYLVANIA 229 



each case has certain indeterminable factors. The necessary defi- 

 ciencies, however, only make the more serviceable any deductions 

 that can be drawn from such data as are available, if they can be so 

 handled as to extend their significance. It has seemed possible 

 therefore that a discussion based on data that have even serious 

 shortcomings may have some value. It is only because it is hoped 

 that this might prove true that it has been decided to carry out these 

 studies on the present observational data. 



The tract of the Appalachian Mountains most readily accessible 

 for dip studies is that which lies along the main line of the Pennsyl- 

 vania Railroad between Tyrone, in Blair County, and Harrisburg. 

 From Tyrone to Mount Union the railroad, following the Juniata 

 River, crosses the folds in a fairly straight line at right angles to their 

 strike and affords a very good section. But just beyond Mount 

 Union the Juniata turns sharply to the northeast and runs for twenty- 

 five miles parallel to the strike, as far as Lewistown. This offset 

 necessitates a division of the whole cross-section into two parts. From 

 Lewistown to Harrisburg the folds are crossed generally at right 

 angles to the strike with the exception of a few minor curves. This 

 constitutes the second portion. Inasmuch as the second portion 

 commences near Lewistown at the point corresponding to that where 

 the first section left off near Mount Union, it would seem that no 

 great structural error is introduced by ignoring the shift and uniting 

 the two separate parts into one section. 



At every available rock-outcrop along each of these lines, the dip 

 of the strata was read with a clinometer compass, using the telegraph 

 poles which are set thirty-eight to the mile as a means of locating the 

 stations. For rapid work of this sort the spacing of the telegraph 

 poles may be used so as to give quite closely the distance intervening 

 between the locations of outcrops. Allowance, of course, must be 

 made whenever the railway-line crosses the folded structure obliquely 

 and curvingly, instead of normal to the strike. Wherever there 

 occurred sudden changes in the angle of dip, or small local folds, 

 diagrammatic sketches were made of the rock-face, and on these 

 sketches the clinometer readings were recorded at the appropriate 

 points. Where there were good outcrops the details of the folded 

 structure were readily discerned, but unfortunately there were 



