THE CAMBRIAN PERIOD. 251 



of beds in the Upper Cambrian of Wisconsin. In this state the for- 

 mation had an original thickness of a thousand feet or so (less than this 

 at its original margin), though within the area where it is now exposed 

 its thickness is usually less. It is made up chiefly of sandstone, is 

 indeed generally known as the Potsdam sandstone; but it is much less 

 uniform from bottom to top than this name might suggest. The 

 following section * represents the general sequence of beds in the south- 

 central part of the State: 



Feet. 



6. Sandstone (Madison) 35 



5. Limestone, shale, and sandstone (Mendota) 60 



4. Sandstone, calcareous , . 155 



3. Bluish shale, calcareous 80 



2. Sandstone, slightly calcareous 160 



1. Very coarse sandstone, non-calcareous 280 



The second division of the series from the bottom suggests either 

 that conditions were such that shell-bearing life was rather more abun- 

 dant than at first, or that the waters were more highly charged with 

 lime carbonate. The third division, the bluish shale, shows that the 

 sediments accumulating where this section occurs were finer than those 

 which preceded or followed. This might mean either that the water 

 was here temporarily deeper, or that the land from which the sedi- 

 ments were being derived was lower, so that finer sediment only was 

 supplied. The fourth division represents a return to the conditions 

 favorable for the accumulation of sand, while the fifth indicates that 

 conditions had so changed that accumulations of sand alternated 

 with those of mud and limestone. Toward the close of the period, 

 there was a return to conditions favorable for the deposition of sand. 

 Even this does not represent the full measure of the variations, for 

 within each of the several subdivisions mentioned there are minor 

 variations. These are often brought into prominence when the rock 

 weathers (Fig. 100). Similar variations are repeated in many places 

 for comparable thicknesses of rock, and each change in sedimentation 

 probably indicates a change in the geography of the period. 



1 Geology of Wisconsin, Vol. I, p. 259. 



