44 A NATURALIST IN THE GREAT LAKES REGION 



top of another but with many interruptions on the southern 

 flank of this continental nucleus (map, Fig. 28, compare Fig. 37). 

 The underlying formations would not everywhere be the same 

 because in some places the surface rock is one of those deposited 

 early, and consequently has few members of the series below it, 

 and in other places (those farther south) , the rock is a later forma- 

 tion and has many members below it. Often one or more 

 members of the series may be absent at a given locality if that 

 spot was land when said member was being deposited as might 

 readily be the case since the sea was so incessantly changing its 

 shape from period to period. 



When such a term as the Niagara limestone is used it does not 

 imply that the formation is necessarily homogeneous; it may 

 consist of successive strata of different sorts of rock or, if of the 

 same sort, the succeeding layers show by the unlike animal and 

 plant fossils they contain that they belong to different successive 

 fragments of geological time. The Niagara limestone, the bed 

 rock of the immediate Chicago region, was laid down in this 

 interior sea without any very violent interruptions, but the out- 

 let of the sea was shifted several times during its deposition so 

 that the fossil remains in successive levels of the formation are 

 quite unlike, some having affinities with arctic forms, some with 

 tropical, and some with those of the Atlantic and Pacific. The 

 formation is thus divisible into several distinct sets of strata. 

 In the Chicago region there are at least the lower coral beds, the 

 upper coral beds, the reef limestones (Fig. 29), and the Guelph. 

 Similarly, the Potsdam sandstone exposed farther north consists 

 of three quite distinct sets of strata, the lowest sandstone that 

 has a thickness of about a thousand feet, the Dresbach shale and 

 sandstone, and the Franconia sandstone, the two latter each 

 about two hundred feet thick. The names applied to the succes- 

 sive rock formations encountered in Illinois and adjacent terri- 

 tory are shown in the table at the close of the preceding chapter. 



The character of the rock often tells much of the conditions 

 prevailing at the time of its formation. Beds of sand and gravel 



