THE LOWER SILURIAN ROCKS. 



583 



vale, coming to the surface over a belt of country several miles in width. Even east of 

 the limestone edge, a considerable area of the adjoining portions of Otsego and Foun- 

 tain Prairie is eroded down to the level of the Madison sand beds, and yet surrounded 

 entirely by the higher formations. Such an effect is a peculiar one, and takes place only 

 where the inclination of the surface bears such a relation to the changing inclination of 

 the strata as indicated in Fig. 37. The Lower Magnesian limestone caps many of the 

 outlying bluffs east of the Wisconsin, forms the summit of most of the higher ridges in 



FIG. 37. 



r\ $"* wr -feUWR5rrc- - - 



2^grg^- --SSSOg^ 



DIAGBASI SHOWING HOW AN ABBA o* A LOWER FORMATION MAY BE ENTIRELY SURROUNDED BY 



A HIGHEK ONE. 



western and southwestern Sauk, and is the surface roclfc nearly everywhere in Columbia 

 county, east of the limestone edge. In this latter district, however, it is overlaid by the 

 St. Peters sandstone and Trenton lime-stone, in southeastern Hampden, and southern Co- 

 lumbus, over a small area in eastern Fountain Prairie, and over a much larger one in 

 northeastern Courtland and eastern Randolph. The St. Peters sandstone occurs also in 

 several patches in southwest Arlington, lying upon the irregular upper surface of the 

 Lower Magnesian, and forms the upper part of a remarkable bluff in the northeast part 

 of Weet Point. The characters of the several formations arc indicated in the following 

 detailed descriptions. Their thickness, relative positions, etc., are shown by the sections 

 of Plates XX, XXII and XXIII of this volume, and those attached to the Atlas Plate of 

 Area E. 



Beginning our detailed descriptions in the northeastern comer of Columbia county, 

 we note first, on Sec. 31, T. 13, R. 12 E., Randolph, two quarries on the Mendota and 

 Madison beds, on the walls of a ravine at the head of Duck creek. The quarry on the 

 west wall of the ravine shows the following section: 



MEXDOTA BEDS. 



I. 



Ft. In. 



Very fine-grained, yellowish-brown, calcareo-silicious rock (1206); only 

 slightly arenaceous in feeling, but leaving a residue, on treatment with 

 acid, of 69.03 per cent., which consists of exceedingly fine, white, angu- 

 lar quartz; thinly and regularly laminated, the layers running from one 



to four inches 10 .. 



II. Very fine-grained, close-textured, yellow-and-purple-blotched, calcareo- 

 argillaceous rock (1207); the yellow parts like I, the dark-colored a 

 sort of clay-shale, occasionally finely laminated, but not in distinct layers; 

 residue on treatment with acid, 44.53 per cent.; layers, two to eight 



inches 6 .. 



III. Very fine-grained, milk-white, silicious rock, without calcareous admix- 

 ture; non-arenaceous; similar to II of next section 6 



