THE LOWER SILURIAN ROCKS. 541 



Fine-grained sand layers; of rajying colors, in upper portions; witii gaps of 10 to 

 70 feet; including at base some quite coarab-grained sand layers; all formed of 



rolled quartz, and aD entirely non-calcareous 310 



The same continued in tlie Artesiaji well 385 



Red shale I5 



Total 710 



At the smnrait of the Elephant's Back, fragments of trilobites 

 occur in the loose sand rock, and the horizon may be the same as that 

 on the top of Roche k Oris. The occurrence here of the same red 

 shale as observed in the Madison wells is worthy of notice. The 

 same layer has been reached by Artesian borings in other parts of the 

 state. 



Returning now to the Baraboo ranges and passing northward from 

 their western ends along the west side of the district, we find in the 

 town of Reedsburg, Sauk county, the Lower Magnesian, Madison and 

 Mendota, with their usual characters. In the same town, at points 

 some miles apart, exposures of a bed of red shale are to be seen whoso 

 horizon is 140 feet below the Mendota base. Further north, in north- 

 western Juneau, a high, ridgy country is met with, carrying sand- 

 stone at high levels, in what would be expected to be the Lower 

 Magnesian horizon. This, however, appears to be due to a thicken- 

 ing of the Madison beds, since the Lower Magnesian is found capping 

 a few very high points, and the Mendota beds below continue recog- 

 nizable. In southwestern Juneau county, on the inner side of the 

 high ground bordering the central plain, are numbers of isolated 

 sandstone outliers. Some of these show a bed of red shale and soft 

 greensand, which appears to be the same as that seen in the town of 

 Reedsburg. This greensand layer, about 130-140 feet below the 

 Mendota, is the lowest seen anywhere in the Potsdam series of Cen- 

 tral Wisconsin. 



Still further north, the country is generally eroded well down into 

 the middle of the Lower sandstone, so that the Archaean rocks are pot 

 very far beneath the surface, which they come nearer and nearer to, 

 towards the north. Over much of Portage, Wood, Jackson and Clark 

 counties, the thickness of the sandstone caunot be more than 25 to 

 60 feet. In places in this region, the sandstone lying within 20 to 40 

 feet of the crystalline rocks is a much indurated, coarse, white rock, 

 which yields a valuable quarry stone, and appears to occupy the same 

 horizon as a similar rock in Waushara and Marquette counties. It is 

 probably to be referred nearly to the middle part of the Lower sand- 

 stone. The sandstone immediately in contact with the crystalline 



