1884.1 ' ibO [Stevenson. 



on the easterly side of Tuscarora mountain in Franklin county of Pennsyl- 

 vania. During the folding, the rock yielded and was broken into great 

 masses which moved on themselves so as to permit the folds to be made. 

 These planes of fractures are the "dry seams" of the tunnel-men. 



This condition is equally well marked in the great sandstones of the 

 region. No traces of it appear on the surface, aside from the presence of 

 occasional planes along which silicious matter appears to have been de- 

 posited and which have a slickensided surface. But in the great tunnels 

 now driving by the South Penn. Railroad Company, the true condition is 

 sufficiently clear. The crush broke the sandstones into enormous wedges ; 

 during the folding these were rubbed against each other so as to polish the 

 faces and to fill the crevices with clay. These are a source of danger and 

 anxiety to those driving the tunnels and they render arching necessary 

 where the rock is such that arching would be thought wholly unneces- 

 sary. 



The fracturing in the Utica shales is even more noticeable, for there the 

 masses are much smaller and the fragments which fall from a tunnel roof, 

 even when the beds are standing at ninety degrees, vary from five to five 

 hundred pounds weight, while the slates show the results of the terrible 

 pressure by their slickensided surfaces, separated by not more than an 

 inch. Yet despite this terrible pressure, the black shales of the Utica ap- 

 pear to have lost none of their carbonaceous matter and, on the freshly 

 fractured surface, do not diflFer from shales of the same age in Central New 

 York where the disturbance has been practically nothing whatever. 



The sandstones of the Vespertine and Upper or White Medina, in like 

 manner, show no change whatever. The White Medina in its upper por- 

 tion resembles quartzite, but this is due to conditions during its deposit. 



Effect of Contact or Proximity of Ervptive Bocks. 



The influence of eruptive rocks is as variable as that of the other agen- 

 cies. Near the head of East river in Colorado, not far from the head of 

 Rock creek, a narrow dike of trachyte cuts the Colorado shales, and they 

 have been changed into true slate for many feet on each side. At other 

 localities on the same stream, enormous trachytic overflows, nearly two 

 thousand feet thick, rest on the shales. Metamorpbism extends to a dis- 

 tance of only a few inches. Nor indeed do the shales show any change 

 as the results of the enormous pressure of the overlying trachyte. 



Dikes are very numerous in the San Juan region of Southwest Colorado. 



The eruptive rocks appear to equal the sedimentary rocks in quantity. 

 The effect on the latter is very marked, for in many places the metamor- 

 phosis is so great that one has difficulty in determining that he is not ex- 

 amining an eruptive rock. ^ 



In the vicinity of Old Baldj--, a trachytic mass just north from Cimarron 

 creek in New Mexico, there are many dikes which reach far out toward 

 the east, invading the whole of the Cretaceous from the Dakota to the 

 Laramie, There, Colorado shales have been changed into true slates ; 



