THE LOWER SILURIAN ROCKS. 



539 



which I am somewhat loath to advance, as too bold a generalization 

 from the facts in hand. It is not impossible that the true explana- 

 tion may lie in the supposition that during the deposition of the Pots- 

 dam series the quartzite' ranges, being high islands and reefs in the 

 ancient seas, received synchronous littoral depositions at high and ab- 

 normal altitudes, the sand and bowlders for these depositions coining 

 from the wear of the quartzite itself. 



Leaving now the Baraboo region, and proceeding northward along 

 the eastern side of the district, we find, everywhere in the neighbor- 

 hood of the escarpment that forms the western edge of the main area 

 of the Lower Magnesian, the same succession of layers as seen along 

 the Wisconsin in Columbia and Sauk counties, i. e., Madison and 

 Mendota beds, underlaid by 100 to 150 feet of calcareo-arenaceous 

 layers, and these again by brown and white non -calcareous sandstone. 

 This succession holds true at least as far as Waupaca county, and 

 probably further than this. West of the escarpment, in Wausharn, 

 Marquette and Columbia counties, the country surface is generally 



FIG. 34. 



IDEAL STRUCTURAL SKETCH, SHOWING POSSIBLE RELATIONS OF THE HORIZONTAL FORMA TIOJIH IM 

 THE VICINITY OP THE BARABOO QUARTZITE RANGES. 



well down in the Potsdam, and much drift-covered. In central Wau- 

 shara county, however, are some high hills reaching into the limy 

 beds just beneath the Mendota, and showing the normal succession 

 of layers; whilst in the very northeast corner of Marquette county, 25 

 miles from the boundary of the main Lower Magnesian area, are two 

 isolated bluffs, capped by that formation, which show also the usual 

 layers. It is thus evident that for all this region there are no de- 

 partures from the Madison section. At several points in Marquetto 

 and Waushara counties quarries are opened in beds that lie about 200 

 feet below the Mendota, and yield a much indurated, white sandrock, 

 which is occasionally quite coarse, and is made up of nearly glassy 

 quartz pebbles. 



Proceeding now westward into Adams and Juneau counties, we 

 find again some apparently abnormal occurrences. One mile west 



