Age of the Quartzites^ Schists, Mc.j of Sauk Co. 135 



excellent opportunity for examination. Tlie total thickness 

 seen was about twelve feet, tlie layers varying in thickness 

 from a few lines to four or five inches. Very thin films of a 

 talcose material sometimes appear between the layers. Directly 

 above this schist, I found a horizontal undisturbed sandstone, 

 laid open for some distance by quarrying. The beds are gen- 

 erally a foot or two in thickness. In the loose pieces near by 

 is found Scolithus linearis. The sandstone is, of course, the 

 Potsdam of the surrounding valleys. Section 2 will serve to 

 give a clear idea of the structure of this bluff. 



The narrow detached ridge just to the westward, represented 

 on the map, is also made up of horizontal Potsdam sandstone. 

 There are many other such detached ridges along the Baraboo 

 valley, bearing the same relation to the quartzite ranges, and 

 showing the same horizontality of strata. 



The following arguments in favor of the priority of these 

 rocks to the Potsdam period will, I think, after what has been 

 said, be admitted as valid. 1 give them in the order in which 

 they became apparent to me. 



1st. The limited area of disturbance ; the undisturbed Potsdam 

 and Calciferous strata being found north, south, and between 

 the ridges, and in close proximity to them. 



2d. The absence of any anticlinal axes. Dipping as the rocks 

 do uniformly to the north, in order to place them in the Pots- 

 dam category, we must imagine a metamorphism of the strata, 

 accompanied by a great fault, having on one side the unchanged 

 sandstones, and on the other the tilted quartzites and schists, 

 an idea new, I think, to geology. 



3d. Tlie occurrence of rounded pebbles of quartzite in the con^ 

 glomerate on the south side of tlie south range. To suppose this 

 conglomerate, which by its fossils is unmistakably Potsdam, 

 to be of the same period as the quartzites below, we must sup- 

 pose that period to have lasted long enough to cover the depo- 

 sition of the quartzites as sandstones, their metamorphism, and 

 the rounding of the pebbles by beach action, before the forma ' 



