THE LOWER SILURIAN ROCKS. 607 



and shelving ledges of brownish, friable St. Peter's are frequent on the valley sides, and 

 isolated bluffs and towers of the same rock are to be seen at several places within the 

 valleys themselves. One of these towers, on the S. W. qr. of the S. E. qr. Sec. 11 

 Primrose, known as the Devil's Chimney, is circular in section, 60 feet high, 50 feet in 

 diameter on the top and 40 feet at the bottom. The isolated bluff on the N. E. qr. of 

 the S. W. qr. of Sec. 28, Springdale, is 100 feet high, 100 yards in diameter at base and 

 20 on top. 



On tlie Wisconsin river slope the exposures and quarries, which are numerous, are 

 chiefly in the Potsdam, Mendota, Madison and Lower Magnesian. The Trenton is 

 quarried, however, on the N. E. qr. of Sec. 18, Middleton, at the top of a high bluff, 

 showing 90 feet of St. Peters (658) as represented in Fig. 51. 



FIG. 51. 



SECTION ACROSS THE VALLEY OP BLACK EARTH CREEK. 

 Vertical scale 200 feet to the inch. Horizontal scale 400 feet to the inch. 



The Trenton at this place (659, 660, 661) contains numerous casts of the following 

 fossils: Pet raia corniculum, Strophomena, Cypricarditesventricos:isRapliistomalenticu- 

 lare, Trochonema umbilicata, Murchisonia bicincta, M. tricarinata, Pleurotomaria Nz- 

 soni, Oncoceras pandion, and Orthoceras anellum. The Trenton shows also in a small 

 quarry at the top of the bluff on the N. W . qr. of Sec. 28, Beiry, far away from any 

 other Trenton area. 



