118 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



K-L 13. BLACK HILLS, SOUTH DAKOTA. 



The Cambrian Deadwood formation of the Black Hills is described by Darton ^^^^ 

 in detail in his paper on the geology of the hills. A convenient summary of the facts 

 is given in a later paper /^^'^ as follows : 



Deadwood formation. — The outcrop of this formation encircles the Black Hills, appearing 

 usually in the base of the great infacing limestone escarpment. It lies on a relatively smooth 

 surface of Algonkian rocks, although there are many local irregularities of shore lines and beach 

 phenomena. The rocks are mostly sandstones and sandy shales, with frequent occurrences of 

 basal conglomerate. In the southern hUls the thickness varies from 4 feet to 50 feet in greater 

 part, and the principal material is a coarse dark-brown massive sandstone. To the northward 

 the formation gradually thickens, apparently by the addition of higher beds comprising dark- 

 gray shales, mostly sandy, and beds of sandstone. In the region about Deadwood, where the 

 formation attains its greatest thickness of over 400 feet, it comprises about 30 feet of basal con- 

 glomerate overlain by 30 feet of coarse dark-brown sandstone; 200 to 400 feet of gray shales, with 

 layers of flaggy limestone, limestone conglomerate, and sandstone; a conspicuous member of 

 hard massive sandstone 5 to 12 feet thick; and at the top 20 to 45 feet of green shales. The lime- 

 stone conglomerate is a very characteristic rock, consisting of flat pebbles and flakes of limestone 

 more or less thickly sprinkled with glauconite grains, and is of the intraf ormational type. 



Throughout its course the formation contains fossils of which the following middle Cambrian 

 forms have been reported by C. D. Walcott: Obolus, Hyolithes, DiceUomus, Asaphiscus, Ole- 

 noides, Ptychoparia, and Acrotreta. 



K-L 15-16. MINNESOTA, WISCONSIN, AND MICHIGAN. 



Sardeson '■"'' suggests that the "New Uhn" quartzite of southern Minnesota 

 may be of Cambrian age, as it contains pebbles of " quartzite conglomerate like the 

 rock (Huronian?) which is in situ at Redstone." The well records,^®* however, 

 which have been carried across southern Minnesota, from the type region of the 

 Sioux quartzite (Algonkian) to and beyond New Ulm, have served to correlate this 

 quartzite with the Algonkian Sioux quartzite. 



The Cambrian and lower Ordovician strata of the upper Mississippi Valley have 

 been divided by recent writers into the following formations, all of which are well 

 established in the literature of several States : 



Ordovician: 



St. Peter sandstone. 



Praiiie du Chien group (formerly called ' ' Lower Magnesian limestone " ) : 

 Shakopee dolomite. 

 " New Richmond sandstone. 

 Oneota dolomite. 

 Cambrian: 



Jordan sandstone. 

 St. Lawrence formation 

 Dresbach sandstone. 

 Shales and sandstones. 



The Cambrian strata in southern Wisconsin were described as the " Potsdam 

 sandstone" by Chamber lin,^^^ who stated the synopsis of characters thus in part: 



Rocks mainly light-colored sandstone in central and southern Wisconsin and red sandstone 

 in the Lake Superior region, but embrace some beds of limestone and shale. Maximum known 

 thickness about 1,000 feet. 



