THE BASEMENT COMPLEX OF THE MARQUETTE DISTRICT. 555 



Basic igneous rocks intrude in an intricate manner Ijotli the Upper 

 Marquette and the Lower Marquette series. 



The aim of the following paragraphs is to briefly sketch the history of 

 the district. 



THE BASEMENT COMPLEX. 



The oldest rocks of the Basement Complex are thoroughly crystalline, 

 foliated schists and gneisses. A close tield and laboratory study has failed 

 to detect in them any evidence of sedimentary origin. If any detrital rocks 

 are included in the Basement Complex, they have been so profoundly meta- 

 morphosed as to have lost all evidence of their origin. These gneisses and 

 schists have been cut by various igneous rocks at different epochs. The 

 latter occur both in the form of great bosses and in dikes, sometimes cutting, 

 sometimes parallel to, the foliation of the rocks. In some cases the niiraber 

 of intrusive belts of granite parallel to the schistosity is so large and they 

 are so narrow as to give very numerous interlaminations of schist and granite 

 within a short distance. 



In the area of the Northern Complex there were volcanic outbursts, 

 and a vast series of lavas, agglomerates, gi-eenstone-conglomerates, and 

 tuffs were piled iqi. By far the greater part of the volcanic material is of 

 an intermediate or basic character. While tlie material is undoubtedly 

 a surface deposit, a search year after year in the field has failed to reveal 

 any decisive evidence of arrangement by water. The deposits are strictly 

 volcanic. After the great lava beds and the vast masses of tuffs were piled 

 up there were granitic, syenitic, and diabasic intrusions, for bosses of these 

 rocks and dikes from them cut through the volcanics. 



After, and also perhaps during, the building up of the volcanic seines 

 the Marquette district was deeply truncated, as a consequence of which 

 many of the different varieties of rocks composing the Basement Complex 

 appeared at the surface. The coarse granites must have formed as deep- 

 seated rocks, and the foliation of the schists must have formed far below 

 the surface; such rocks could have reached the surface only by long- 

 continued denudation, Avhich removed mountain masses of materials. The 

 process continued until the Basement Complex had no great altitude, for 



