HYPOTHESES EXPLAINING THE FACTS 675 



dimension of the magma were too small, sufficient differential pressure 

 would not be developed to force the superjacent rocks aside. As this 

 dimension increased, the rate of rise would be more rapid. We know too 

 little of the effect of differential pressure on rocks subjected to the tem- 

 perature and pressure at great depths to know what pressure would be 

 necessary to cause a magma to rise, so the best idea we can express is 

 that the horizontal dimension of the fluid mass was probably much less 

 than the vertical. 



With this necessarily somewhat vague picture in mind — of the size 

 and shape of the rising magma — let us start with the Archean baselevel, 

 take up the various events in order, and point out how each might be, 

 and possibly is, related to the great intruding mass. 



One of the early surface manifestations of the rise of such an extremely 

 large mass of magma would be the elevation of the surface, the magma 

 floating the surface rocks up. This might well occur long before any 

 other surface evidence of a molten mass below. Assuming the position 

 of the magma to be under the axis of the present syncline, a gentle 

 doming of the Archean baselevel would take place, with resultant slopes 

 down to the northwest and southeast, the long axis of the elevation being 

 parallel to the present synclinal axis. This elevation would revive 

 erosive processes and sediments would be deposited in the water-filled 

 depressions on the flanks of the dome. These depressions can well be 

 imagined to be produced by the "undertow" of rock flowage — away at 

 the top, down at the sides, and inward toward the bottom of the magma. 

 These first sediments form the Huronian formations, chiefly the quartz 

 slates, underlying the iron formation in the Mesabi and Gogebic iron 

 districts. The inference from Palms ripple-marks that the shoreline 

 extended in a northeastern direction fits with the tilting ascribed to the 

 magma. 



The next major event is the deposition of the Mesabi and Gogebic iron 

 formations. Van Hise and Leith, in Monograph LII of the United 

 States Geological Survey, ascribe the origin of these formations to mag- 

 matic water which contributed the iron and silica in solution to the 

 water bodies in which the formations were deposited. In my own study 

 of the Gogebic such a source seems to be the only one that offers an 

 adequate explanation. These silica-iron-bearing waters may be consid- 

 ered to be the first surface contribution of the great mass of magma we 

 are considering — a pre-volcanic emanation of vast volumes of solution 

 that gave the first surface relief from pressure below. This relief might 

 well result in a pause in the elevation of the surface and a more or less 



