VARIATION IN METAMORPHISM. 919 



extremely metamorphosed district or area by connecting- it with areas which 

 are less metamorphosed. 



These principles are well illustrated at various places. The Penokee 

 series of the Lake Superior region is very little metamo: phosed in the east- 

 ern part, but is much more metamorphosed in the western part. A still more 

 striking illustration is found in the Marquette district. In the eastern and 

 central part of the district the rocks are so little metamorphosed that there 

 is no difficulty whatever in determining the origin of the formations. But in 

 the western part of the district, especially in the closely mashed Republic 

 tongue, the three unconformable series there existing — the Archean, the 

 Lower Huronian, and the Upper Huronian — are so mashed and metamor- 

 phosed as to appear completely conformable, and one might conclude that 

 here is an inseparable series. However, by tracing the formations along 

 the strike until they connect with the less metamorphosed portions in the 

 central and eastern parts of the district, one is certain that in this tongue 

 there exist three unconformable series. 



A third excellent illustration of the change of metamorphism along 

 the strike is furnished by the Ocoee series, which in its northeastern part, in 

 east Tennessee, is but semimetamorphosed, and there it is easy to recognize 

 the original character of the formations. As these formations are traced to 

 the southwest they become more and more metamorphosed, until they are 

 schists and gneisses, and then it is difficult to discriminate between the 

 sedimentary and igneous parts of the series and to separate the Ocoee 

 series from the older series unconformably below. 



(2) Rapid change in metamorphism across the strike is frequently as 

 characteristic as is gradual change along the strike. Of course in some 

 cases the change across the strike is gradual, but in many cases it is very 

 abrupt. Thus the rocks on the crown of an arch or at the bottom of a 

 trough may be only partly metamorphosed, while the same formations on 

 the limbs of the folds may be profoundly metamorphosed. This is due to 

 difference in the amount of shearing in different parts of the folds." In 

 other cases the abrupt change in amount of metamorphism is due to the 

 fact that orogenic movement may die out rapidly across the strike or 

 change in its character. As a consequence, rocks on a hill may be folded 



"Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept. 

 U. S. Geol. Survey, pt. 1, 1896, pp. 598-600. 



