998 THE GEOLOGY OP MINNESOTA. 



[Consequences of hypothesis. 



into jaspilyte. Near the shore such glassy lavas were eroded by wave action and 

 distributed so as to form conglomerates and sandstones. Such action would have 

 distributed lavas wholly silicified as well as those which were yet glassy, and the 

 detritus of both would necessarily mingle with detritus from the Archean. Such 

 lavas would exhibit great contortion, and in places great brecciation, the same as 

 later lavas, and these breccias must have been mingled sometimes with the products 

 of detrital action. After prolonged activity of the volcanoes most of the deposits and 

 of the lavas which were submarine would be permeated by secondary silica, but 

 carbonate of iron would permeate the mass where carbonic acid had freer access, as 

 in the lagoons into which streams drained from the land surface to the north. 



Consequences of this hypothesis. 



If this hypothesis be true, certain results follow, bearing on the structural 

 and chemical geology, and on the future economic development of the ores. 



1. We probably see only the northern border of the iron-bearing rock. The 

 original basic lava, as well as the resultant volcanic sand, would probably accumu- 

 late in greater quantity to the south of the volcanic belt than toward the north. We 

 know nothing about the location of the original craters. They may have been to the 

 north of the present line of strike of the Animikie or further south. Wherever they 

 were, it is evident that the accumulation of the larger deposits of volcanic sand 

 would be to the southward of the larger deposits of lava. Where the lava form of 

 the iron-bearing rock subsists still in the Mesabi range, as through much of its eastern 

 extension it is reasonable to look for globular taconyte toward the south further, 

 even if the surface rock be of the slates of the Animikie. 



2. This volcanic epoch may have a deep-seated connection with the Cabotian 

 of the Keweenawan. 



3. It renders it probable that the igneous layers in the black slate about Gun- 

 flint lake are not all intrusive sills, but may be in part cotemporary with the 

 fragmentals. 



4. An intimate alliance in mode of origin is shown between the ores and much 

 of the ore-bearing rock, of the Vermilion and Mesabi ranges. This brings to mind 

 the views of M. E. Wadsworth, who first argued for the igneous origin of the jaspilyte 

 of the Marquette region. His evidence consisted of what he considered intrusive 

 relations with the adjacent rock, and he supposed the present nature of the rock is 

 that which it had when molten. The writer has shown elsewhere that both these 

 ideas are incorrect, and that much of the Vermilion jaspilyte is a bedded oceanic 

 precipitate, probably analogous to the bedded flints of the Animikie. However, Dr. 

 Wadsworth is to be credited with the first suggestion as to the manner of origin of 

 the igneous portion of the jaspilyte. This includes that which is much contorted 



