STRUCTURAL GEOLOGY. 



Metamorphism of the fragmental rocks.] 



doubtless other minor folds, and some of these are known to exist in the region 

 between Vermilion lake and Eainy lake, but the rest of the Archean portion of the 

 state is so deeply buried under the drift sheet that it is impossible to indicate, even in 

 a general way, the distribution of the Archean fragmentals. At certain points it is 

 known that rocks of this character appear in other parts of the state. Much of the 

 Archean in Carlton county is of this fragmental character, sometimes bearing a con- 

 siderable proportion of volcanic debris. Similar rock underlies the western part of 

 Morrison county, while the central and eastern parts of that county are underlain 

 by metamorphic conditions of the elastics as at Little Falls and by granite. The 

 rocks that appear along the Minnesota valley between New Ulm and Big Stone lake 

 are also wholly crystalline, consisting of gneisses and schists, cut by granite and 

 diabase, but no sufficient examination has been made to warrant the statement that 

 clastic characters do not also occur. Such characters are most likely to remain in 

 the rocks in outcrop farthest toward the southeast, the intensity of recrystallization 

 apparently increasing toward the northwest. 



Metamorphism of the fragmentals of the Archean. It is not intended at this place 

 to enter upon the question of the dynamics and processes of metamorphism. That 

 question is more fully treated under the head of petrographic geology, parts ii and iii, to 

 which the reader may be referred for many details of the microscopic changes wrought 

 in the clastic rocks by dynamic forces and by mineralizing waters. It is only 

 necessary here to call attention to such grand structural features as the state presents, 

 which are dependent upon, and accompany, the principal belts of metamorphic change. 



The centres of intense metamorphism are the points at which igneous rocks 

 have penetrated the strata, whether granite or gabbro. At the borders of the 

 granitic bosses the elastics are converted to gneisses and mica schists. In general 

 such areas are more elevated than the non-metamorphic areas, but there are important 

 exceptions. A glance at the geological map of the state shows the positions of these 

 granitic and gneissic areas. In general they have a northeast and southwest direction, 

 and between these hardened belts lie non-metamorphic rocks. It appears that very 

 early in the geological history of the state and of the northwest, a system of crustal 

 folding was imprinted on the earliest rocks, and that this was attended by the appear- 

 ance of igneous irruption at the fissure-lines which were produced at the upward 

 flexures. The rocks that were thus flexed and broken, so far as they appear by the 

 study which we have given to the state of Minnesota, consisted of greenstones, quartz- 

 porphyry and their attendant elastics, of which the former may be assumed to repre- 

 sent the oldest rocks, and perhaps resulted in a large measure from the consolidation of 

 the surface of the molten globe. These rocks, especially the acid fragmental, have been 

 subjected to earth-movements and widespread metamorphism which have given rise 



