THE LOWER SILURIAN ROCKS. 587 



white, incoherent Madison sandstone. Another and much larger Mendota quarry is oil 

 the south side of the bluff in the S. hf of Sec. 18, T. 10, R. 8 E. Here art some ten 

 feet of very regularly bedded, yellow, sandy limestone, the layers below heavy, above 

 thin and shaly, with fine large impressions of Dlcellocephalus Minnesotensis. 



The very prominent isolated bluff on the N. E. qr. of Sec. 20, T. 10, R. 7 E., shows 

 the following section at its north end : 



LOWER MAGNESIAN. 



Feet. 



1. Grassy slope, without exposure 35 



2. Coarse, crumbling, brownish sandstone 2 



3. Slope without exposures 18 



4. Brownish-yellow, rough, open- textured limestone, somewhat crystalline; contain- 



ing cavities with calcite crystals, numeorous red quartzite pebbles and green- 

 sand grains 10 



MENDOTA AND POTSDAM. 



5. Coarse, brownish sandstone, in perpendicular ledges 10 



6. Slope without exposure 85 



7. Friable, non-calcareous light-colored sandstone 15 



8. Sand-covered slope without exposure 60 



Height above road at foot 235 



FIG. 40. 

 Section 25 Section 25 



S.I/V. DiTV&vm of Rectum H.ZTE. 



SECTION OF KINOSLET'S BLUFT, LODL 



Horizontal scale 1,0)0 feet to 1 inch. Vertical scale 309 feet to 1 inch. Figures indicate altitudes 



in feet above Lake Michigan. 



The occurrence of a thin layer of non-calcareous sandstone within the Lower Magne- 

 sian is unusual, but this is not an isolated instance. The limestone No. 4 is interesting 

 because of its similarity to the rock from Eiky's and Wood's quarries in the Baraboo 

 valley, the accurate determination of whose stratigraphical position meets with some 

 difficulties, and because of its somewhat peculiar characters as compared with the 

 ordinary Lower Magnesian, the base of which formation there can be no doubt that it 

 marks. From a similar, and equally prominent bluff, on the south side of the samo 

 section, the profile of Fig. 41 is taken, running across the Wisconsin to the quartzito 

 range of Merrimac. 



Gibralter Bluff is the name given to the bald cliff of St. Peters sandstone which sur- 

 mounts the western end of a large outlying area of limestone-capped bluffs, in Sees. 17 



