STRUCTURAL GEOLOGY. 53 



The Puokwungc conglomerate.] 



on the Chippewa river. Here it underlies a massive quartzyte, and reaches the 

 thickness of 300 feet.* It has been fully described by the Wisconsin Geological 

 survey.f It is probably the same as that described by Van Hise at the west of 

 Agogebic lake, where it contains rolled pebbles of quartz-porphyry and certain 

 phases of basic eruptives and debris from the Archean. + 



The later Keweenawan fray mental rocks were at first interstratified with sheets 

 of lava of diabasic nature. They are frequently conglomeratic, but consist largely 

 of debris from the cotemporary trap. These beds of loose, fine, red conglomerate 

 or of sandstone, are apt to be thickly sprinkled with crumbling laumontite, an 

 additional circumstance which increases their friability. Hence, they are frequently 

 seen along the lake shore, as at the mouth of Manitou river, and west from Little 

 Marais, forming perpendicular cliffs capped with the next overlying lava sheet. 

 Such conglomerate is to be seen in the St. Louis valley, above Fond du Lac, 

 interstratified with red sandstone and red shale, these beds in the aggregate reaching 

 a thickness of over 800 feet. Here no known trap-sheets are interbedded. However, 

 in the record of the deep well at Short Line park, a series of trap flows separates the 

 red sandstones and conglomerates from the basal, coarse Puckwunge conglomerate, 

 reaching a thickness of 217 feet. The whole formation dips, at and above Fond du Lac, 

 at an average angle of about six degrees toward the south-southeast. The uppermost 

 layers become more siliceous and serve as a good building stone. 



As the eruptive forces died out the sandstone became still purer, and indeed 

 quite pure, forming first the Hinckley sandstone, quarried in the gorge of Kettle 

 river in Pine county, and later the sandstones of the St. Croix valley, which are 

 fossiliferous and alternate with magnesian shales and magnesian limestones, thus 

 introducing gently the fauna of the Upper Cambrian. 



During this later part of the Keweenawan in the northwest, both during the 

 continuance of gentle or local eruption, and after its entire cessation, the whole 

 region was undergoing a slow settling, bringing the oceanic waters over more extended 

 areas of land, and burying under the later sandstones, not only the old Archean 

 rocks, as seen in Ca'rlton county (vol. iv, p. 16), but also concealing from sight the 

 older beds of the Keweenawan itself, as exhibited at Taylor's Falls. It thus appears 

 that the Manitou portion of the Keweenawan eruptive rocks are interjected in the 

 midst of a period of sandstone accumulation. They would thus be expected to 

 occur somewhat locally and at irregular intervals, and about the peripheries of the 

 eruptive centre they would be found to diminish gradually as to thickness and to 



* Notes on the Gctiloiiy of U'ixninxiii. Wisconsin Aeiutemy nf Scicnrc and Arts, vol. iii, p. 45. 



t Geology of Wisconsin, vol. iv, p. 575. 



I The Penokee iron-bearing series of Michigan and Wisconsin . Man. xir, U.S. Geol. Survey, -p. 461 



S\ According to L. L. HUBBARD, state geologist of Michigan, this subsidence began early in the Keweenawan, and was 

 continued during the accumulation of the 8,000 feet of coarse conglomerate on Kcwecnaw point. 1'rocmilimjx Jsike Superior 

 Minimi institute, ii, p. 96, 1894. 



