268 F. W. SARDESON ^A CORROSION CONGLOMERATE 



one here described. There is in fact a variety of conglomeratic deposits 

 comprised in the Cambrian and Ordovician in this region. Sea-beach 

 pebbles, probably also river gravel, certainly some lag-gravel, with "drei- 

 kanter," and some marine corrosion pebbles, besides reef breccia, or con- 

 glomerate, characterize different ones of the known conglomerate beds, 

 and they show that there was a diversity of conditions under which con- 

 glomerates were formed. Several of the conglomerate beds appear to be 

 problematic, and they may be considered more especially at some future 

 time. With them are placed for the present certain false conglomerates. 



FORMATIONAL RELATIONS 



A brief outline description of the corrosion phenomena and the reason 

 for calling the deposits intraformational corrosion conglomerate may be 

 given again here. At Saint Paul, Minnesota, there are eight zones at 

 which evidence of corrosion is found in a vertical section of about 110 

 feet. Of the eight zones two (numbers 2 and 3, figure 1) are merely 

 blackened corroded top faces of limestone strata. Two others (numbers 

 1 and 7) are corrosion surfaces, with associated black pebbles, while the 

 rest (numbers 4, 5, 6, and 8), including the one especially considered here 

 (number 8), are conglomeratic only. The eight zones are in the several 

 distinguishable beds or faunal zones of the Galena-Trenton series next 

 above the Saint Peter sandstone. As properly named, the Platteville 

 limestone includes beds numbers 1 and 2, the Decorah shale includes beds 

 numbers 3, 4, and 5, and the base of the Galena limestone, which is rather 

 shaly here also, is bed number 6. Thus the corrosion zones 1, 2, 4, 5, 6, 

 and 8 are intraformational, and numbers 3 and 7 are pactically so, too, 

 since the formational distinction is mainly technical. Further, the zones 

 numbers 2, 3, and 7 can be nothing other than the result of corrosion, as 

 shown by their features, which are as follows : 



, CORROSION SURFACES 



The faces of certain limestone strata are uniformly blackened by a 

 stain which penetrates with a diminishing intensity an inch or so down- 

 ward into the fresh limestone. The black-stained surfaces lie, as a rule, 

 along seams between strata, but belong distinctly to the top of the sub- 

 jacent stratum and never to the bottom of the superjacent one. Where 

 the black surface or seam cuts through fossils, as frequently occurs, there 

 are remaining parts of them in the limestone below the seam, but none 

 above it, just as must be the case where the top of a limestone had been 

 corroded or eroded before the succeeding stratum was deposited. There 

 is in fact an unconformity on a small scale at each corrosion surface. 



