

walcott.] SUMMARY UPPER MISSISSIPPI VALLEY. 331 



pods, and brachiopods. The succession of beds is, according to Prof. 

 T. C. Ghamberlm, as follows: 1 



On the southern side of the Archean Island the lower part of the formation usually 

 consists of coarse quartzose sand, of an exceedingly open, porous nature, with but 

 little aluminous or ferruginous and almost no calcareous matter. Higher in the 

 series the sandstone becomes finer grained and the accessory substances named more 

 abundant. Somewhat above the middle of the series a stratum of shale occurs, at- 

 taining a known thickness of 80 feet. This is not everywhere present and seems to 

 be mainly developed at some distance from the ancient shore line. It appears to 

 indicate that for a time there was a deepening of the waters, admitting of the accu- 

 mulation of fine sediment, except near the shore, where the deposit of sand continued. 

 Above the shale the sandstone, reaching a thickness of 150 feet, is again found. This 

 is medium or coarse grained and slightly calcareous. It, in turn, is overlain by a 

 deposit of associated shale and limestone (the Mendota limestone), which attains a 

 thickness of 35 feet in tho vicinity of the lake from which it derives its name. These 

 beds indicate a modification of the conditions of deposition, such as to permit not 

 only the settling of line sediment, but the accumulation of calcareous mud as well. 

 The latter was doubtless derived from the calcareous remains of life, since the sea 

 then swarmed with living organisms whose shells and skeletons are found entombed 

 in the strata. The frequency of broken and worn fragments implies that the greater 

 portion were ground to powder, forming the calcareous flour that subsequently hard- 

 ened into limestone. These beds appear to point quite surely to a moderate deepen- 

 ing of the waters. 



Overlying this impure limestone is a third and thinner bed of sandstone (the Madi- 

 son) with which the Potsdam series closes. This, on the whole, is finer grained than 

 that below, and is bound more firmly together by cementing material, which is 

 mainly a calcareous aud ferruginous infiltration. The thickness of this bed is about 

 30 feet. 



In eastern Wisconsin the formation is divided by Prof. Chamberlain 

 into six parts, as follows, beginning at the top: 



Feet. 



Sandstone (Madison) 35 



Limestone, shale, and sandstone (Mendota) 60 



Sandstone, calcareous 155 



Bluish shale, calcareous 80 



Sandstone, slightly calcareous 160 



Very coarse sandstone, noncalcareous 280 



The thicknesses- given are subject to considerable variation. As a general rule 

 they grow less toward the northeast. Where the total thickness of the formation is 

 reduced by the inequalities of its Archean bottom it is by the loss of the lower 

 members of the group aud not by the thinning of all. 2 



The Upper or Madison sandstone is described as a rather coarse- 

 grained, thick-bedded, compact, but soft, slightly calcareous, light-col- 

 ored sandstone. In its upper portion, immediately beneath the lower 

 Maguesiau limestone, it is at most localities coarse, and the topmost 

 layer is often broken up and mixed with calcareous material, giving it 

 acoarsely brecciated structure. The subjacent limestone (Mendota) con- 

 sists of a group of alternating strata of arenaceous magnesian lime- 

 stone, sandy calcareous shales, and shaly and calcareous sandstones. 1 



1 General Geology. Historical Geology. Paleozoic Era. Geology of Wisconsin. Survey of 1873-79, 

 vol. 1, 1883, pp. 121,122. 



[Geology of eastern Wisconsin. 1 Chapters 7 and 8. Lower and Upper Silurian. Geology of Wis- 

 consin, Survey of 1873-79, vol. 2, 1877, pp. 259, 260. 



J Op.cit.,p.260. 



