182 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



shale, which is made up largely of microscopic unicellular algae and which contains 

 as much as 20 per cent of heavy oil. 



Some beds of the Galena contain abundant organic remains, but the main body of the 

 dolomite carries few recognizable fossils. Locally, however, where the basal limestone and 

 shale we're deposited, as shown in the first two sections of the Platteville limestone given in the 

 description of that formation, they are highly fossiliferous. 



The Galena and Platteville form a thick dolomite-limestone series, which is of much impor- 

 tance in the upper Mississippi Valley. The lithologic features of the two formations here 

 described are those which are common to the lead and zinc district. Outside of this district 

 the characteristics vary somewhat, and toward the west the Galena becomes less dolomitized, 

 especially in its lower part. 



In the lead and zinc district the Platteville, commonly known as "Trenton" in the reports 

 on this district, is essentially a nondolomitic formation, while the Galena is a dolomite. If this 

 lithologic distinction should be used as a means of separating these formations in areas lying 

 farther west the thickness of the Platteville would be there greatly increased. Recent work, 

 however, has shown that the Platteville is a weU-defined formation of comparatively uniform 

 thickness, possessing distinctive faunal characteristics; that, at least locally, it is separated 

 from the Galena by a period of nondeposition ; and that in certain localities, especially at places 

 west of the Mississippi, the Gralena is in considerable part nondolomitic. 



Upon the Galena dolomite lies a shale formation, which has been described in reports of the 

 Iowa Geological Survey under the term Maquoketa, a name derived from Little Maquoketa 

 River, on which it is typically exposed. The same formation is known in the Wisconsin State 

 reports as the Cincinnati shale. * * * 



The shale ranges in thickness from 160 to 200 feet and in the region south of this area is 

 more than 200 feet thick. 



^ 't* T* T" 'I' 'P M* T* -|C 



The Maquoketa shale in this area can be arranged roughly into the tliree characteristic 

 divisions described below: 



General divisions of Maquoketa shale. 



Feet. 

 3. Argillaceous and calcareous shale, in beds becoming thicker and possibly dolomitic toward 



the top, about 35 



2. Plastic blue and green shale and clay, with some indurated fossiliferous bands near the top, 



about 100 



1. Drab and blue thin fissile shale, interbedded with thin layers of fossiliferous argillaceous 



limestone; near base is a thin, hard conglomerate of fine chert and phosphatic pebbles in a 



ferruginous matrix 60 



The Maquoketa shale in general throughout the upper Mississippi Valley forms a markedly 

 fossUiferous horizon. There is at the very base of the formation a thickness of 2 to 5 feet of 

 shale which contains rather abundant fossUs. * * * At the top of the formation there is 

 another horizon, more frequently exposed, that affords fossils, at some places in great abun- 

 dance. * * * This association of species indicates that the beds are of Richmond age. 



* * * Definite evidences of stratigraphic unconformity or of an erosion interval at the 

 base of the Maquoketa have not been observed during the survey of this area. The change 

 from the dolomite beds of the Galena to the shale is fairly abrupt, and mention has been made 

 of a smaU. conglomerate that occurs locally in the basal shale, but the sedimentation planes on 

 the two sides of the contact line are parallel, as far as seen. The evidence, so far as it goes, does 

 not indicate any marked physical break between the formations, although the fossils would 

 indicate a period of nondeposition. 



