INTEODUCTION, XXIIl 



FOLDING OF THE ARCHEAJST AND HURONIAN SERIES. 



The Crystal Falls district had now been an area of deposition for a 

 very long- time, and a great tliickness of sediments had accumulated. A 

 profound physical revolution next occurred, the greatest since Archean 

 time. The region was raised above the sea and was folded in a most com- 

 plex manner. As a consequence, the more conspicuous folds vary from a 

 north-south to an east-west direction. The closer folds in the northeastern 

 part of the area are nearly north-south. In the central paii of the area the 

 closer folds strike northwest-southeast. In the eastern and southeastern 

 parts of the district the closer folds are nearly east-west. All of these 

 folds, however, have steep pitches. It therefore follows that the region 

 was subjected to great compressive stresses in all directions tangential to 

 the surface of the earth, and that the yielding was mainly in one direction 

 here and in another there, although on every fold there is evidence of yield- 

 ing in two directions at right angles to each other. Some of the folds are 

 very close, as in the case of the Huronian area between the two Archean 

 ovals in the northeastern part of the district, and in the Felch Mountain 

 range. In other areas — as, for instance, in the Crystal Falls syncline — 

 the major fold is somewhat open. However, upon the open folds are super- 

 imposed folds of a higher order, so that the detail structure is very compli- 

 cated. So far as known, the district has nowhere been faulted. 



Subsequent to or during the late stage of this time of folding there 

 was a period of great igneous activity, probably contemporaneous with the 

 Keweenawan. At this time there were introduced into both the Lower and 

 the Upper Huronian rocks vast bosses and numerous dikes. The intrusives 

 vary from those of an ultrabasic character, such as peridotites, tlu-oiigh those 

 of a basic character, such as gabbros and dolerites, to those of an acid char- 

 acter, siTch as granites. These intrusives, while altered by metasomatic 

 changes, do not show marked evidence of dynamic metamorphism — there- 

 fore the conclusion that they were introduced later than the period of intense 

 folding, already described. 



A few illustrations are mentioned. The Archean and other great 

 massifs are less profoundly altered than are the softer and weaker deposits 

 of the Huronian. In these more rigid formations, such as the granites and 

 quartzites, all phases of alteration by granulation and recrystallization are 



