EXTENT OF FOLDING AND SUBSEQUENT EROSION 448 



suffered before the deposition of the higher beds. That Eichmond strata 

 (chiefly the Juniata red shales and sands representing Eichmond beds in 

 the east) were involved in this folding is suggested by the succeeding 

 events. This narrows the period of deformative movement down to the 

 interval between early or middle Eichmond and some time before the 

 beginning of Salina time. The Susquehanna section indicates that this 

 period must be narrowed still further, and that the folding took place 

 during the otherwise almost unrepresented interval between Ordovicic 

 and Siluric time. That this period was one of almost if not quite com- 

 plete withdrawal of the sea from the interior of the North American 

 continent is indicated by the widespread disconformity between Eich- 

 mond and Clinton formations. This disconformity is marked in the 

 Upper Mississippi region by pronounced indications of erosion of the 

 older beds, especially the Maquoketa shales, which are classed by Schu- 

 chert as Middle Eichmond. As has been shown by Weller 61 and others, 

 these Maquoketa beds mark a widespread transgression of the sea during 

 later Eichmond time, resting generally with a disconformity on Trenton 

 (Galena, etcetera). There is therefore much reason for believing that 

 the entire Upper Eichmond was deposited over the northwestern part of 

 the United States. That this Eichmond transgression was coincident 

 with the spread of the Juniata fan is not improbable and is not at all 

 inconsistent with the character of these deposits. The absence of highest 

 Eichmond in the Upper Mississippi section is to be accounted for by 

 subsequent erosion during the emergence which preceded the Lower 

 Siluric (Magaran) transgression of the sea. That this interval was 

 coeval with the folding and subsequent partial erosion of the Ordovicic 

 strata seems probable and is indeed suggested by the character of the 

 early Siluric deposits. An interesting fact, throwing some light on the 

 character of the interior at this time, is found in the eroded surface of 

 the Maquoketa shales of the eastern Wisconsin region and its relation to 

 the succeeding deposits. The erosion plane is covered by shale pebbles 

 which have a pronounced glossy surface, intimately suggestive of desert 

 varnish. These pebbles form a pavement on which the early Siluric 

 strata rest. These latter begin in at least one case with an oolitic iron 

 ore, the form and structure of which is such as to suggest dunes of 

 oolite similar to those now forming on the border of the Great Salt Lake. 

 Subsequent replacement converted these oolites into iron ore' 5 '-' (figures 



61 Stuart Weller: Journal of Geology, vol. 15, pp. 510-525, 



03 See A. W. Grabau : Physical and fauna! conditions, etcetera, in "Outlines of geo- 

 loglcal history," i>. tc>. For the discussion of the general problem of oolite formation, 



see Principles of Stratigraphy, chapters ix and xl. 



