THE LOWER SILURIAN ROCKS. 553 



The foot of the cliff is evidently very close to the top of .the Mad- 

 ison sandstone, whose first exposure, however, is 15 feet below. Al- 

 though the foreign impurities increase slightly in quantity down- 

 wards, we find no distinct evidence of a gradation into the sandstone 

 below. In fact the whole cliff shows a nearly uniform material, the 

 differences being but slight between the several layers. 



North of Dane county the Lower Magnesiau has the same general 

 characters as described, with some local variations; but no scheme of 

 elementary stratification for these districts has been made out. In 

 the high prairie country of southern Columbia county the formation 

 attains a thickness of 120 to 140 feet or more, the highest beds being 

 generally very cherty, or even replaced bodily by chert. In central 

 and northern Columbia the lowest layers have lost their irregularity 

 of bedding and rough- texture, and have become very evenly be.dded 

 and closely granular, at the same time showing little or no sandy ad- 

 mixture, and no passage downwards into the Madison sandstone, 

 which itself continues non-calcareous upwards to contact with the 

 Lower Magnesian. Along the western side of the district, in west- 

 ern Sauk county, the same lack of gradation downwards is generally 

 to be noticed. In a large region lying south of the Baraboo quartz- 

 ite ranges, small pebbles of red quartzite are frequently found in the 

 Lower Magnesian. 



The irregular upper surface of the Lower Magnesian, already 

 mentioned, is one of the most striking features of the formation. 

 The first demonstration of the existence of such an eroded surface was 

 made by Prof. Chamberlin, in eastern Wisconsin, but since then 

 numerous confirming facts have been collected in other parts of the 

 state. The valley of Sugar river, and its numerous branch ravines, in 

 the towns of Verona, Montrose, Primrose, Springdale and Cross 

 Plains, cut down to the Lower Magnesian, the St. Peters sandstone 

 forming the steep valley sides. At numerous points in these valleys, 

 exposures of the Lower Magnesian are found at higher levels than 

 those of the St. Peters, and under such circumstances that they can- 

 not be regarded as proving a distinct and hitherto unrecognized layer 

 of limestone; for they are often near to large sandstone ledges, which 

 rise continuously from lower to higher levels than those at which the 

 limestone is seen. A still more striking proof is found in the patches 

 of St. Peters sandstone that are to be seen lying directly in the hol- 

 lows of the Lower Magnesian, in the southern part of the town of 

 Arlington, Columbia county; whilst the evidence is perhaps even 

 stronger in the case of Gribralter Bluff, in the town of West Point, 

 Colnrnbia county, where a vertical cliff 135 feet high, of St. Peters 



