List of localities of rocks examined, with brief descriptive notes.... In the typical Huronian of Lake Huron.. In the iron-bearing series of Marquette, Mich In the iron-bearing series of the Penokee region of Wisconsin. In the slates of the Saint Louis River, Minnesota.... In the quartzite formation of the Baraboo region of Wisconsin... In the quartzite formation of Southern Minnesota In the Animikie series of Northern Minnesota, and the Thunder Bay In the folded schists of the national boundary line, north of Lake ILLUSTRATIONS. To face page. PLATE I.-Huronian quartzite of East Neebish Rapids, Saint Mary's River.. FIG. 1. Single grain of the sandstone intercalated in Huronian quartzite of East Neebish Island. Enlarged 67 diameters. No. 17 of FIG. 2. Another grain from the same sandstone, drawn from a balsam FIG. 3. Single grain of the Huronian sandstone of Spurr Mountain, Mich. FIGS. 4, 5, 6. Single grains from the Potsdam sandstone of New Lisbon, FIGS. 7, 9. Single grains of the Potsdam sandstone of the quarry on the Torch Lake Railroad, Keweenaw Point, Michigan. No. 90 of the list, p. 40. Enlarged 67 diameters. FIG. 8. Portion of a thin section of the same sandstone, showing how nucleal fragments and enlargements polarize together. Enlarged 67 diameters. The black spaces are holes in the section. PLATE III.-Thin sections of quartzites FIG. 1. Quartzite crust of Saint Peter's sandstone, Arlington Prairie, FIG. 2. Arenaceous quartzite, Animikie series, Portage Bay Island, Min- PLATE IV.-FIGS. 1 and 2. Thin sections of red quartzite, Animikie group, same section in ordinary and polarized light. Enlarged 39 PLATE V.-Thin sections of sandstone and quartzite..... FIG. 1. Indurated Saint Peter's sandstone, Gibraltar Bluff, Columbia FIG. 2. Huronian quartzite of Neebish Island, Saint Mary's River. No. 1 13 15 23 25 9 To face page. 29 PLATE VI.-Thin sections of quartzite, &c...... FIG. 1 (upper half). Huronian quartzite from gannister quarry, south side of Carp River, near Marquette, Mich. No. 22 of the list, p. 28. Polarized light. Enlarged 31 diameters. FIG. 1 (lower half). Huronian quartz-schist from near Marquette, Mich. No. 27 of the list, p. 29. Polarized light. Enlarged 36 diameters. Designed to show schistose structure and induration by independently oriented quartz. FIG. 2 (upper half). Red quartzite of Prairie River Falls, Minnesota. No. 58 of the list, p. 35. Enlarged 31 diameters. Polarized light. Drawn to show the enlargement of a complex grain. FIG. 2 (lower half). Cherty Potsdam sandstone, Westfield, Sauk County, Wisconsin. Polarized light. Enlarged 35 diameters. cherty matrix holds grains of quartz which have received enlargements. (194) A I. ENLARGEMENTS OF QUARTZ FRAGMENTS AND GENESIS OF QUARTZITES. BY R. D. IRVING AND C. R. VAN HISE. GENERAL CONSIDERATIONS. In his address before the Geological Society of London, delivered February 20, 1880, Sorby describes sands whose grains are bounded. externally by crystalline faces, but have in the interior the ordinary rolled grains, the crystalline faces having been produced by a secondary deposition of quartz upon the irregular surfaces of the original grains. He shows also that the quartz coatings of these grains are in perfect "optical and crystalline continuity" with the interior fragments, each original fragment having thus been changed to a definite crystal. He states further that he had examined crystalline sands from the sandstones of various ages "from the Oolites down to the Old Red," and that they are commonly little coherent, but that in some specimens "a number of grains may often be seen cohering more strongly than the rest, and these show clearly that the cavities originally existing be tween the grains have been more or less completely filled with quartz. Moreover, on carefully examining the less coherent grains by surface illumination, we can see not only the planes and angles due to unimpeded crystallization, but also more or less deep impressions due to the interference of contiguous grains, thus proving conclusively that the deposition of crystalline quartz took place after the nuclei were deposited as a bed of normal sand. The very imperfect consolidation sometimes met with is, perhaps, not so very surprising when we reflect on the very small coherence of many large quartz crystals which are yet in close juxtaposition. However, it does seem probable that this crystallization of quartz sometimes contributes very materially to the cohesion of the grains in hard and compact quartzites. In such examples as the Ganuister of the South Yorkshire coal-field we can see in a thin section that the grains fit alongside one another in a very striking manner, and it is only by extreme care that good proof can be obtained of the actual deposition of quartz between them. However, in the case of a highly consolidated sandstone from Trinidad the proof of the deposition of quartz is as complete as possible; 1 Proc. Geol. Soc. Lond., 1880, p. 62. |