ILLUSTRATIONS 



Plate I. Colored map showing the distribution of pre-Caiubrian and other rocks in the Lake 

 Superior region, and the geographical relations of the Crystal Falls district of 



Michigan to the adjoining Marquette and Menominee districts of Michigan 11 



II. Topographical map of the Crystal Falls district of Michigan, including a portion of 



the Marquette district of Michigan In pocket. 



III. Geological map of the Crystal Falls district of Jlichigau, including a portion of the 



Marquette district of Michigan In pocket. 



IV. Portion of a geological map of the Menominee iron region, by T. B. Brooks and C. E. 



Wright 18 



V. Generalized sections to Illustrate the stratigraphy and structure of the northwestern , 



part of the Crystal Falls district of Michigan 28 



VI. Generalized sections to illustrate the stratigraphy and structure of the southern part 



of the Crystal Falls district of Michigan 28 



VII. Generalized columnar section 30 



VIII. Map of a portion of the Crystal Falls district, showing in detail the glacial topog- 

 raphy and illustrating the development of the Oeer River 32 



IX. Sketch of the Mansfield mine as it was before it caved in, in 1893 66 



X. A, Reproduction of the weathered surface of a variolite; B, Reproduction of the 



polished surface of a variolite 110 



XI. Colored reproduction of an ellipsoid, with matrix, from an ellipsoidal basalt 116 



XII. Mount Giorgios, viewed from its west flank, in April, 1866, illustrating the charac- 

 teristic block lavas, from Fouque's Santorin et ses Eruptions, PI. VIII 120 



XIII. Reproduction iu colors of a basalt tulf 140 



XIV. Idealized structiiral map and detail geological map, with sections, to show the dis- 



tribution and structure of the Huronian rocks in the vicinity of Crystal Falls, 



Michigan 160 



XV. Portion of Brooks's PI. IX, Vol. Ill, Wisconsin Geological Survey 172 



XVI. Detail geological map of the vicinity of Amasa, Michigan 176 



XVII. Detail geological map of the vicinity of Crystal Falls and Mansfield, Sheet 1 178 



XVIII. Detail geological map of the vicinity of Crystal Falls and Mansfield, Sheet II 178 



XIX. A, Inclusions in a fractured quartz phenocryst ; B, Quartz phenocryst with rhombo- 



hedral parting 268 



XX. ^, Micropoikilitic rhyolite-porphyry; i?, Micropoikilitic quartz-porphyry 270 



XXI. ^, Very fine-grained micropoikilitic ryholite-porphyry viewed without analyzer; B, 



Very fine-grained micropoikilitic rhyolite-porphyry viewed with analyzer 272 



XXII. ^, Perlitic parting in aporhyolite; £, Perlitic parting in aporhyolite 274 



XXIII. A, Schistose rhyolite-porphyry ; B, Aporhyolite breccia 276 



XXIV. ^, Schistose rhyolite-porphyry; i?. The same viewed between crossed nicols 278 



XXV. ^,Amygdaloidal texture of basalt; J?, Amygdaloidal vitreous basalt 280 



XXVI. A, Amygdaloidal vitreous basalt; B, Amygdaloidal vitreous basalt showing sheaf-like 



aggregates of feldspar 282 



XXVII. A, Reproduction in colors of amygdaloidal basalt; B, Pseudo-amygdaloidal matrix of 



ellipsoidal basalt; C, Water-deposited pyroclastio 284 



XXVIII. J, Fine-grained basalt with well- developed igneous texture: J?, Illustration of the 

 obliteration of the igneous texture of a basalt by secondary products when viewed 



between crossed nicols - 286 



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