REVIEWS 173 



8. The great quartzite formation, which cuts quite a figure in the geology ot Wis- 

 consin and Minnesota, is nonconformable upon the Animikie, and is a member of the 

 fragmented beds of the Keweenawan. This has been named "Sioux quartzite," "Bar- 

 aboo quartzite," and "New Ulm quartzite." It is that which contains the red pipestone 

 (catlinite) in southwestern Minnesota. It is the western representative of the Pots- 

 dam sandstone, of Potsdam, N. Y. This quartzite seems to be the representative of 

 the Middle Cambrian, as the Beckmantown is of the upper Cambrian. 



9. The origin of the Mesabi iron ore is referred to a greensand, which has been 

 altered, affording iron ore by concentration of the iron in certain favorable positions. 

 Cotemporary with this alteration was a concentration of silica, and this was increased 

 by oceanic precipitation. The original greensand was found to become pebbles, and 

 to increase into angular masses that were neither sand nor pebbles, but rather breccia. 

 These breccia masses have at first an amorphous crystalline texture and grade into a 

 form of the iron-bearing rock which was named " taconyte," and the whole was referred 

 to volcanic action, being different forms of suddenly cooled volcanic glass and rhyo- 

 lite, broken and distributed by beach action. While this volcanic debris was under- 

 going this transformation, great quantities of silicia were set free from the glass ; but 

 this silicia immediatly saturated the debris, producing spotted jasperoid, taconyte, and 

 sedimentary jaspilyte. 



Having reached this result on the Mesabi Range, it opened the door to the under- 

 standing of the iron ores of the Vermilion Range, and at once the rhyolitic forms and 

 all the igneous associations of those ares with basic igneous rocks were elucidated, thus 

 -confirming Wadsworth's idea of the igneous origin of the jaspilytes of the Marquette 

 region — rather the igneous origin of the rock which later was changed into jaspilyte- 



10 and II. After prolonged field examination, the Minnesota Survey reached the 

 ■conclusion that the granites of the Archaean grade into gneiss, the gneiss into mica- 

 ceous gneiss and mica schist, and finally into less and less metamorphic rocks that show 

 a plain fragmental structure and sedimentary origin. There was found no exception 

 among the Archtean granites. The granites are of two dates of formation — one at 

 the close of the Lower Keewatin, and one at the close, or after the close, of the Upper 

 Keewatin. A later granite, associated with the gabbro, and grading into it, is of the 

 Keweenawan, and another did not spring from a deep source, but is a surface prod- 

 uct of metamorphism carried to the extreme of fusion, on clastic materials that were 

 later than the basal. greenstones. Adventitiously they form intrusions in some of the 

 later (and especially into the clastic) greenstones, but they are not known to penetrate 

 the oldest greenstones. Tentatively the alkaline and the acid siliceous elements in 

 these early sediments were supposed to have been derived from the atmosphere, as 

 the basal crust could not have afforded them. 



In the same manner the gabbro, which becomes acid and grades into syenite, was 

 derivdd from the metamorphism and fusion of the greenstone with their clastic varia- 

 tions. Diabase was found to pass insensibly into gabbro ; but, on the other hand, it 

 is also certain that it was the original form of all igneous greenstones, and that it must 

 have had, and still has, a deep-seated source. 



These belts of intensest metamorphism, whether productive of granite or of gabbro, 

 have a parallelism with each other, and with the northwestern rim of the great syn- 

 ■clinorium of the basin of Lake Superior, marking successive continental folds, in har- 

 mony with a system which continued through Archaean and Taconic time, and even 

 into the Upper Cambrian. 



