920 A TREATISE ON METAMORPHISM. 



and in the next valley be unfolded. They may be very closely folded in 

 a valley and on the mountain beyond be in a horizontal position. The 

 rocks exposed may be strongly metamorphosed by mashing in a mountain 

 mass, because deeply buried during the movement, and along the adjacent 

 area merely be fractured, because in the zone of katamorphism during the 

 movement, Since the amount and kind of metamorphism are so directly 

 a function of the deformation and the depth, the rocks where greatly 

 deformed or deeply buried may be profoundly metamorphosed, and those 

 which are little deformed or near the surface may be comparatively little 

 altered. Failure to understand these principles has led to many mistakes 

 in stratigraphy. Some illustrations of the rapid change across the strike, 

 and of the mistakes in stratigraphy which have resulted, are as follows : 



(a) The Taconic Mountains are immediately adjacent, across the 

 strike, to the Hudson River series of New York and Connecticut. The 

 latter formations are little metamorphosed. The rocks in the Taconic 

 Mountains are schists and gneisses. It was therefore concluded by 

 Emmons and by others that the rocks of the Taconic Mountains were 

 metamorphosed before the Hudson River rocks were deposited, and are 

 therefore much older. Hence Emmons gave the Taconic rocks the name 

 Taconic series. The entire Taconic controversy, which lias cut such a 

 figure in America, arose from this mistake. It was only when Dana, 

 followed by Pumpelly and those who worked with him, appreciated that 

 there might be very rapid chang-es in metamorphism across the strike that 

 it was ascertained that the unmetamorphosed Hudson River rocks 

 immediately to the west of the completely metamorphosed schists and 

 gneisses of Greylock and the other Taconic Mountains are parts of the 

 same sedimentary formations. 



(b) Another region which very well illustrates the rapid change in 

 metamorphism across the strike is that of the Great Valley and the Unaka 

 Mountains, in the southern Appalachians. The Unaka Mountains are 

 largely composed of the Ocoee series, which is of lower Paleozoic age, and 

 which in the southern part of the region is completely metamorphosed. 

 The metamorphism is much greater than in the rocks of the Great Valley 

 immediately adjacent, also Paleozoic. In the southern part of the region 

 the much metamorphosed Ocoee is thrust over the little altered rocks of the 

 Great Valley. 



