MINERALOGY AND PETROLOGY. 997 



Additional facts.] 



gave origin to the poikilitic arid globular pyroxenes. In either case the rock is a 

 mixture of ferruginous (originally clastic) taconyte with elements that are distinctly 

 igneous or metamorphic, suggesting close relations in the origination of the two kinds. 



9. Lastly, the crucial evidence was found at a point about a mile west of Gun- 

 flint lake, described and illustrated by figure 54, on page 951 (Nos. 1897, 2052 and 2053). 



A sideritic rock (No. 2052) makes quite a display in a conspicuous hill. It is 

 the iron-bearing member or its representative, and shows red because of the oxidized 

 surfaces. It is broken by later diabase intrusions which afford a varied manner of 

 contact on the sideritic slates, the diabase being coarsely porphyritic. This sideritic 

 rock passes with the dip southward and becomes a breccia of confused composition. 

 In the hill (No. 2052) it is somewhat actinolitic. At the railroad cut (figure 54) it 

 is almost wholly of actinolite, but still has so much siderite that it turns yellow with 

 iron rust (Nos. 1897, 2053) and it contains many quartzose flinty masses, some being 

 two feet long. In the matrix of this quartzose breccia the above sideritic rock 

 becomes not only actinolitic but diabasic and glassy (No. 2053). So far as can be 

 judged from the field-notes, and from the thin sections examined, the sideritic iron- 

 bearing member passes through an actinolitic phase and afterward acquires the form 

 of a basic volcanic glass, from which, on cooling, were formed small spherulitic 

 segregated masses like those of the lava sheet of Grand Portage island (No. 544). 



It is apparent, therefore, that the iron-bearing member has various phases, 

 especially when it is not economically productive, viz.: 



(a) It is a basic glass. 



(b) It is a jaspilyte, or silicified basic rhyolitic lava. 



(c) It is a sideritic rock (not fragmental). 



(d) It is an actinolitic siderite. 



(e) It is a sideryte (clastic or chemical precipitate). 



(f) It is a breccia and a conglomerate of basic glass and of jaspilyte. 



(g) It is a taconyte, or sand of volcanic glass and of jaspilyte. 



(h) It is a nondescript gray, greenish-gray, or often brown, rudely bedded 

 siliceous rock (allied to c). 



All the foregoing anomalous as well as the additional facts may be explained 

 by the following hypothesis: 



A chain of active volcanoes, having explosive emissions, extended across north- 

 eastern Minnesota about where the Mesabi Iron range is found. This was near the 

 shore line of the Taconicocean, and was accompanied by land-locked bays, and perhaps 

 by fresh-water lakes. Such marginal volcanoes had a chemical effect on the oceanic 

 water, causing the precipitation of silica and probably of iron. Its basic lavas and 

 obsidians were attacked by the hot waters and were converted by encroaching silica 



