PALEOZOIC ROCKS FOUND IN DEEP WELLS 537 



soft, but in some layers of slate-Kke consistency. Interstratified 

 with the shale at certain horizons are beds of gray and blue dolomite 

 and magnesian limestone. In northeastern Wisconsin portions of 

 the shale are brown-colored and, when heated, give off a strong 

 oily odor. The correct interpretation of drill samples is hampered 

 by the fact that some have been washed while others have not; 

 the former give quite an erroneous impression of the character of 

 the material. 



In northeastern Wisconsin the following subdivisions of the 

 Richmond group have been distinguished in wells: in ascending 

 order {a) brown shale about 100 feet; {h) blue shale 58 to 105 feet; 

 {c) brown shale, locally gray and slaty 5 to 62 feet; {d) blue shale 

 34 to 105 feet; (e) blue and gray dolomite interbedded with blue 

 shale 35 to 70 feet; (/) blue shale 42 to 90 feet, and {g) gray, blue, 

 and purple shaly dolomite with nodules of chert and gypsum 35 

 to 50 feet. Of the foregoing {a) is known only at Sturgeon Bay; 

 (e), (/), {g), and possibly {d) are exposed at the surface; and (c) 

 is quite irregular in distribution and thickness. The last is readily 

 overlooked as the dried samples are hard to distinguish from the 

 grayish blue shales above and below. The Niagaran dolomites 

 bevel across the top of the group so that as one goes south along 

 the shore of Lake Michigan, the younger series rests upon succes- 

 sively older and older members of the Richmond. Where the May- 

 ville dolomite rests upon the dolomite member {e), the exact plane 

 of contact is difficult to determine. Throughout this district the 

 Richmond group varies from a thickness of about 200 feet near 

 Milwaukee, to over 540 feet in southern Door County. 



In northern Illinois the group is thinner and very variable, 

 both in thickness and in lithological character. Calvin^ describes 

 a full section of the Maquoketa beds in Iowa as, ascending (a) 

 Elgin shaly limestones (limestones, dolomites, and shaly Hmestones 

 interstratified with blue calcareous shale) 70 feet; {h) Clermont 

 shale (blue shale) 15 feet; (c) Fort Atkinson limestone (yellow 

 cherty dolomite and hmestone) 40 feet; {d) Brainard shales (blue 

 shale with limestone beds at top and bottom) 120 feet. Savage^ 



^ Samuel Calvin, "Geology of Winneshiek County," Iowa Geol. Survey, Vol. XVI 

 (1906), pp. 94-109. 



^ T. E. Savage, "Geology of Jackson County," Iowa Geol. Survey, Vol. XVI (1906), 

 pp. 597-609. 



