walcott.] SUMMARY — UPPER MISSISSIPPI VALLEY. 333 



In the Mississippi Valley, in south western Wisconsin, Mr. Moses 

 Strong estimated the total thickness of the Potsdam at 800 to 1,000 feet, 

 the inequalities in the surface of the underlying Archeau rocks being 

 the principal source of its variation. 1 The formation is composed al- 

 most entirely of sandstone. The only exception to this is the stratum 

 of magnesian limestone and shale found in the upper part of the for- 

 mation. 



The strata of the formation are usually composed of fine siliceous sand, generally 

 in small, rounded, water-worn grains of almost every color, the most frequent being 

 the various shades of yellow and red, sometimes green, and often snow white. The 

 strata vary greatly in consistency, aud in different localities the same stratum may 

 present different degrees of hardness. Some of the layers, and especially the white 

 ones, are frequently almost as compact as quartzite, and from this all degrees are 

 found, to a loose, friable sand that crumbles in the hand. * * * The stratification 

 of the Potsdam is very regular and even, and the beds usually lie in a nearly hori- 

 zontal position over large tracts of country. Indeed, so little do they deviate from 

 an apparent level that the dip can not bo distinguished by the eye, but only by a 

 careful measurement. The dip is usually to the northwest, aud seldom exceeds 8 or 

 10 feet to the mile, but more frequently it is less. 2 



The most highly fossiliferous portion of the formation occurs in west- 

 ern Wisconsin. The formation is divided in the lower St. Croix dis- 

 trict, by Mr. L. C. Wooster, into four horizons, as follows : 3 (1) " Eau 

 Claire grit; (2) Eau Claire trilobite beds; (3) Hudson trilobite beds, 

 and (4) line of junction with the Lower Magnesian limestone." In this 

 region it borders upon the metamorphic.and igneous rocks to the nortli 

 and east, and touches the Lower Magnesian limestone on the south aud 

 west. 



The four horizons mentioned are described as follows : 



The Eau Claire grit, exposed at the mouth of Eau Claire River, is a very coarse sand- 

 stone, with Scolithus tubes in one or two of the layers. The rock is so coarse that it 

 has been termed a conglomerate, not without reason. 



The Eau Claire trilobite beds are characterized by several species of trilobites not 

 found at any other horizon, and also by being the lower limit at which brachiopods 

 - were found in the sandstone. These beds likewise mark the lower limit of calca- 

 reous matter in the formation. 



The Hudson trilobite beds lie at the best defined horizon in the sandstone. These 

 beds lie in close relationship to aud between the Lower Calcareous band and the layers 

 which are richest in green sand, and thus can not well be mistaken. * * * The 

 shaly layers below the trilobite beds are intensely green from the presence of green 

 sand (a variety of glauconite), which, so far as known, is true of no other horizon. 



The line of junction with the Lower Magnesian limestone may, in nearly every instance, 

 be recognized by the heavy brecciated beds of limestone just above. But this char- 

 acter may fail in the extreme southeastern border of the district, where it is possible 

 that beds of sandstone are iuterstratified with the lower layers of the Lower Magne- 

 sian limestone, as in eastern Wisconsin. 4 



1 Geological formations. (Geology of the Mississippi, regions north, of the Wisconsin River.) Geol- 

 ogy of Wisconsin, survey of 1873-1879, vol. 4, 1882, p. 45. 



2 Op. cit.,pp. 39, 40. 



3 Detailed geology : [Geology of the lower St. Crox district.] Geology of Wisconsin, survey of 1873- 

 1879, vo.. 4, 1882, p. 109. 



«Op.cit.,p.ll0. 



