PrnhU'iiix of Mesah! Iron Oiv. — WlncJieU. 175- 
guide to the stratigraphist iu attempting to determine the rehitive 
ages of these terranes, for the surface must have suffered a pro- 
found degradation. The gabbro rock, which is by all conceded 
to be an irruptive rock, sho\ys no sign of ever having outflowed 
at the surface of the earth. It is not bedded by amygdaloidal 
partings, nor has it, so far as known, any variable texture due to 
contacting with other older rocks. Yet it comes into contact 
with various older terranes, having crowded backward upon them 
while yet confined within the crust of the earth, without reaching 
the surface. It has been seen overlying the Pewabic quartzyte, 
the Keewatin greenstone and the granitic rocks of the Giant's 
range, but maintaining everywhere a coarse and crystalline texture. 
It seems as if the irruptive movement must have been very slow, 
and that it progressed not forcibly, but as rapidly as the heating 
of the adjacent rocks rendered them more flexible. Subsequent 
to the molten invasion the surface degradation took place reveal- 
ing the deep-seated contacts which we see. It has been the 
writer's opinion that this event of the irruption of the gabbro 
took place immediately after the deposition of the Pewabic 
quartzyte, based on the interbedding of that quartzyte with a 
rock resembling the gabbro, and on the observed immediate over- 
lie of the gabbro on an extensive area of the quartzyte. This 
observed overlie, however, loses its importance when it is learned 
that the gabbro also overlies the Keewatin and the crystalline 
granite of the Giant's range, and the date of the disturbance will 
have to remain, as heretofore, not definitely established. 
In further considering, however, the Mesabi iron ore, certain 
problematic difficulties appear in the way of accepting any of the 
proposed theories for the origin of the ore. These may be 
enumerated : 
1. There is no limestone known in the region, which could be 
considered the parent rock giving rise to this ore by a process of 
substitution, nor has there been any struck by any of the diamond 
drills that have recently been driven through the ore horizon. 
2. There is no known horizon of spathic iron which can be con- 
sidered to have been converted to oxide. 
8. There is no dissemination of carbonate of lime in the form 
of calcite, such as to indicate that the ore may have resulted from 
a substitution of iron for lime. The sparse mingling of minute 
fragments of calcite crystals, in microscopic sizes, with the silica 
