444 THE VERMILION IRON-BEARING DISTRICT. 



It is not supposed that the intrusiou of this enormous mass of lava, 

 probably the greatest known laccolith, occurred within a brief time. It 

 has been noted that this laccolith is 150 miles long and probably thousands 

 of feet in thickness — how thick is entirely unknown. The intrusion of so 

 vast a mass of material must have occupied a very great length of time. 

 The parts earlier intruded were doubtless sohdified long before magma 

 ceased to enter. Thus these latter parts would be found as dikes in the 

 earlier solidified parts. There would be great variation in its coarseness 

 of crystallization. There would be ample time for the various processes of 

 differentiation by fractional crystallization and separation by gravity and 

 other processes, and thus is explained the structural complexity of the gab- 

 bro and its great variation in mineral and chemical character. Perhaps 

 contemporary with the intrusion of the gabbro, perhaps later than it, perhaps 

 in part both, was the gentle tilting of the Keweenawan lavas, the Duluth 

 gabbro, and the Animikie, all together, to the south, under Lake Superior, 

 and the much more pronounced northwest tilting toward the same lake 

 of the Penokee series south of Lake Superior. To' this tilting in opposite 

 directions on opposite sides of the lake is due the Lake Superior Basin. 



It is believed that at the time of the formation of the Lake Superior 

 syncline the Giants range antichnal area was correspondingly upheaved, 

 and that thus the present Giants range was actually created by this 

 movement, although, as has been stated in the previous pages, the location 

 of the range was actually determined possibly as early as the folding 

 following the Archean when the protaxis of the range is thought to have 

 been first formed. Since this earliest time repeated movements of 

 elevation, particularly the one at the close of Lower Huronian time, 

 succeeded by subsidence and erosion, have followed along this old line of 

 weakness. The actual present condition of the range in its minor features 

 is of course due to the erosion and then glacial deposition which have 

 occured subsequent to Keweenawan time and which are briefly outlined 

 in the following pages 



Contemporaneous with and following the intrusion of the Keweenawan 

 gabbro is the peculiar metamorphism which marks the rocks of the Lower 

 Huronian, Upper Huronian, and Archean along its border. It has been 

 noted that the Gunflint formation adjacent to it was changed to a banded 

 granitic textured rock, consisting of iron silicates, magnetite, and quartz. 



