4 6 



Minnesota Academy of Science 



reports of the Minnesota survey, especially volumes four and five of 

 the final report. 



I have already stated that the rocks of the two series are dis- 

 tinguished by their different geographic area and by their contrast- 

 ing stratigraphic attitude. But they are also different in their com- 

 position and internal structure. 



I will show you a few lantern slides that portray the internal 

 structural relations, first, of the rocks of the Vermilion iron range, 

 and of their associated strata. The Vermilion ore is in the bottom 

 of the Archean, the oldest rocks known in the state, and in the en- 

 tire Lake Superior region. If they were horizontal, as they must 

 have been originally, they have been compressed horizontally and 

 folded upon themselves, backward and forward, so as to be re- 

 peated perhaps several times in any section that might be observed 

 in traveling across the folds. 



Fig. 3. — Folded Archean Strata. 



(3) This slide exhibits the top of a fold actually observed in 

 Minnesota, near Burntside lake. If the top of this lot of folded 

 strata were to be cut off horizontally, as we know all the Archean 

 strata have been by the waste of time, and especially by the abra- 

 sion of the Glacial epoch, there would be a series of beds standing 

 vertical, running parallel in belts of outcrop, differing from each 

 other in orderly variations from the center of the fold in opposite 

 •directions. It is only where the rocks are bare over large areas that 

 the identity of strata on opposite sides of such a fold can be traced 

 out and proven, and very seldom has such a succession been proved. 



In this folding and squeezing process changes were wrought in 

 the mineralogical composition of the strata. Heat, derived partly 

 from the friction and partly from the interior of the earth, produced 

 chemical transformations, and new minerals resulted from such a 

 metamorphism, and the sediments became crystalline, sometimes pro- 

 ducing mica and hornblende schists and sometimes gneiss; and, if, 

 in such plastic condition, these recrystalized materials were thrust 

 by pressure into any of the cracks, or were extruded at the surface, 

 they became granites of the various degrees of acidity, or sheets 

 of lava. They formed dikes and bosses and all kinds of irregular 

 masses. If they cooled and solidified without being moved from their 

 places they formed gneiss, which is for the most part simply a sedi- 

 mentary rock re-crystallized where it was first deposited. 



