952 THE GEOLOGY OF MINNESOTA. 



[Siclerite. Dolomite. 



The writer cannot at present explain this curious difference in the transforma- 

 tion from basic lava, but he has surmised that some of the lava may have congealed 

 on a land surface and some beneath the water of the ocean, and that the flow brec- 

 cia(?), in which the matrix is largely siderite, may have been originally a surface 

 lava. (Compare No. 1298.) 



Further, if the igneous rock was poured out on a land surface, it is apparent not 

 only that conditions would be favorable for the local formation of fresh-water pools 

 and sometimes of small lakes, but also that, in case of organic matter flourishing in 

 such pools, there would be a deposition of carbonate of iron which might have 

 enclosed, in the manner represented by plate VI, much of the glassy debris of the 

 lava sheet. On this hypothesis the mass of the lava proper is not changed to siderite, 

 but the chilled upper surface is devitrified and its debris is preserved with its char- 

 acteristic features; about the borders of such lakes, and along the sea beach, much 

 volcanic glass sand must have mingled with the carbonate deposit, and sometimes 

 replaced it entirely. 



The further consideration of this presumed igneous origin of the iron-bearing 

 member of the Animikie is deferred till the discussion of the iron ores and the 

 taconyte. It is intended by the foregoing to call attention to the practically simul- 

 taneous origin of the oxide and the carbonate of iron in the iron-bearing rocks of 

 the Mesabi range, and to the microscopical priority of the oxide. Genetically neither 

 depends on the other, but they had a prior common cause of existence, and took 

 different chemical conditions because of differing environments. This accords fully 

 with the- descriptions above of the relations between these minerals in the Vermilion 

 range. That the ore of the Vermilion range was not derived from siderite is proven 

 by rock No. 1572, since that mineral here not only surrounds and embraces the hema- 

 tite, but forms veins that intersect the red jaspilyte in all directions, one vein being 

 a quarter of an inch wide. 



Dolomite, which is a common amorphous ingredient in the limestones in the 

 southern part of the state, has been found to constitute a dolomyte in one instance 

 (No. 824), where its crystalline form is finely exhibited by nearly every grain. It is 

 the upper portion of the matrix of the upper conglomerate at Taylor's Falls, belong- 

 ing to the Upper Cambrian. 



(2) The Colored Minerals Augite. 



This is the most ubiquitous of the ferromagnesian minerals, especially in the 

 basic igneous rocks. Under this designation may be embraced, in a general way, all 

 the pyroxene which has been included in the foregoing microscopical descriptions, 

 although in a few instances some varieties have been specially noted. These will be 

 mentioned below. 



