393 GEOLOGY OF EASTERN WISCONSIN. 



nent, since, eastward from Wisconsin, there intervenes between the 

 Hamilton strata and the Niagara series, the lower Devonian and the 

 uppermost Silurian formations; while to the westward, in northern 

 Illinois and Iowa, the Hamilton beds rest directlj upon the Niagara 

 group. The intermediate formations thin out and disappear, and, as 

 already indicated, the vanishing edge of the Lower Helderberg strata 

 barely reaches the eastern margin of our state. The mingling of Cor- 

 niferous species from below with Chemung forms from above is not 

 age of the only an interesting fact in itself, but is one whose bearing 

 upon the deposit is most significant and decisive, and establishes the 

 correctness of its reference to the Hamilton period. 



Local Descriptions. The most extensive and important outcrop 

 of this formation is found along the Milwaukee river, in the vicinity 

 of Washington Street Bridge, extending above and below, in Sees. 

 4 and 5, T. 7, E. 22 E. The rock nowhere rises to any considerable 

 height above the river-bed, so that no extensive vertical section can 

 be seen, and the frequent interruptions of the exposure, as traced 

 along the river, prevent any trustworthy correlation of the strata. 

 The lithological characters of the rock at this point are essentially 

 those before given as general characteristics, and this locality may bo 

 regarded as the typical one of the formation. A portion of the layers 

 found west of the bridge are more shaly than the average rock of the 

 formation, and tend to disintegrate somewhat more readily on expos- 

 ure. A stratum found below the bridge possesses a more granular 

 character than the rest of the formation, but the chemical analyses 

 that have been made of the several portions, indicate that these varia- 

 tions are largely of a physical nature, and that the chemical composi- 

 tion is less varying. In the drift lying npon this rock, an abtmdance 

 of black shale is present in thin, fragile, more or less rounded chips, 

 indicating the near presence of the formation from which they are 

 derived, and which may be conjectured to be the overlying black slate, 

 so abundant in other regions. J^o exposures of this rock have, how- 

 ever, been discovered in Wisconsin, but as this region is extensively 

 covered with drift, it is not impossible that they exist. 



ISTearly all the species of the foregoing list have been found at this 

 locality, notwithstanding the limited extent of the exposure, and indi- 

 cate a rich and abundant fauna. The working of the beds, which has 

 been commenced since this collection was made, and the importance 

 and interest which now attaches to the formation will, doubtless, 

 very much increase the number of species now given, and develop 

 specimens which will more fully elucidate the structure of some that 

 are now imperfectly represented. 



