Fauna of the Niagara and TJpijer Silurian Bocks. 113 



>^ 



ON THE FAUNA OF THE NIAGARA AND UPPER 

 SILURIAN ROCKS AS EXHIBITED IN MILWAU- 

 KEE COUNTY, WISCONSIN, AND IN COUNTIES 

 CONTIGUOUS THERETO. 



BY F. H. DAY, M. D. 

 Wauwatosa, Wis., Dec. 37, 1877. 



It is stated as an axiom by high paleontological authority, — 

 that "Since rocks are identified more by their fossil contents, than 

 by their lithological character, a name descriptive of the latter' 

 is of less importance than formerly, when fossils were the sub- 

 ordinate characters of a mass ;" and although paleozoic char- 

 acters have assumed the supremacy over all others in distinguish- 

 ing sedimentary strata, " still the lithological terms must not be 

 overlooked ; for if properly understood, they will be unerring 

 guides in tracing the condition of the surface, for more than hun- 

 dreds of miles in extent." 



Changes in the lithological features of a rock which may render 

 observations unsatisfactory, are accompanied by greater or less 

 variation in the nature of the fossils. It is therefore of the high- 

 est importance in the examination of sedimentary rocks to be gov- 

 erned by three essential facts, which are : 



1st. The lithological character. 



2d. The order of the superposition. 



3d. The contained characteristic fossils. 



By an observance of such precepts geologists have been enabled 

 to form a reliable and a systematic geological history, which is ar- 

 ranged into natural distinctions of ages, periods, epochs, and eras, 

 with the capability to trace from one portion of country to another, 

 through all intricate phases, types and characters, the rocks con- 

 taining remains, images or casts of paleozoic life. 



It is thus we determine the first appearance in the world's his- 

 tory of organized beings, as exemplified in thecommencement of 



