MIDDLE AND UPPER ORDOVICIAN. 181 



The Platteville limestone is noticeably different from the other calcareous members of the 

 Paleozoic section in these quadrangles in that it is largely a pure limestone, or a magnesian 

 limestone, rathei* than a dolomite. In earlier reports on this district this formation was called 

 the Trenton limestone, but the name Platteville has recently been apphed to it, as it is possible 

 that these beds do not represent the exact equivalent of the Trenton in its type locahty. * * * 



The Platteville averages 55 feet in thickness and maintains this average over a considerable 

 part of the area. 



Generalized section of PlatteviUe limestone. 



Feet. 



4. Limestone, principally in thin beds, and shale 10-15 



3. Thin-bedded brittle fine-grained limestone 15-25 



2. Thick-bedded magnesian limestone or dolomite 15-25 



1. Blue shale, at some places sandy - 1-5 



It is at some places difficult to draw the boundary between the Platteville and the Galena 

 on purely lithologic criteria. The contact, however, is probably unconformable, despite its 

 obscurity and the fact that it apparently occurs ia the midst of a shaly unit. A close study of 

 the fossils rarely fails to locate the contact within very narrow bounds, and then the actual 

 physical break between the two formations may in places be determined. At many places the 

 first few inches of the Galena consist of a shale or clay containing more or less clear evidence 

 that it is a deposit which has been reworked by water. At some places it contains fragments 

 of fossils, more perfect forms of which occur in the bed just below. At some locahties this 

 lowest bed of the Galena consists of a yellow to white clay, called "pipe clay," underlain by an 

 inch or two of dark carbonaceous clay. 



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The Platteville is commonly highly fossUiferous, more so than any other formation Avithui 

 the area except possibly the Maquoketa shale. Although practically all the beds of the forma- 

 tion except the basal shale contain fossils in greater or less abundance, certain beds are literally 

 packed with organic remains. The dolomitic second member contains relatively few fossil 

 remains, and at some places this b6d may appear almost barren. A few species range through 

 nearly aU the beds, notably Leperditia fahulites Conrad, which is the most characteristic fossil 

 of the formation. Many other species, however, being more restricted in range, may occur in 

 only one or two of the beds. 



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The Platteville is regarded as the equivalent of the Stones River group of Tennessee and 

 Kentucky, a group that lies lower than the Black River and LowviUe (Birdseye) limestones of 

 New York. 



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LithologicaUy the Galena is a dolomite — a granular, crystalline, coarse-grained porous 

 rock, which weathers into exceedingly rough, pitted, and irregular forms. * * * -pj^g 

 formation exhibits the same lithologic character everywhere except in its extreme upper and 

 lower parts. As a whole it is massive in appearance; the average thickness of the beds is from. 

 1 to 4 feet. Near the bottom an.d near the top, however, thinner beds occur. Chert nodules 

 are common in the middle part. At many places very thin seams or partings of clayey material, 

 a little darker ia color than the main mass of the dolomite, separate the formation into irregular 

 layers. * * * 



The thickness of the formation within this area averages about 235 feet. 



The upper portion of the Galena is thin bedded, earthy, and noncrystalline. 

 Shale occurs as partings, and in the uppermost strata, where the Galena passes into 

 the Maquoketa, it forms the rock and the limestone becomes nodular. Near the 

 bottom of the Galena occurs a so-called oil rock, a finely laminated brown to black 



