TES LOWER SILURIAN ROCKS. 537 



white, incoherent Madison sandstone. Another and much larger Mondota qua^ is on 

 the south side of the blnif in iixe S. hf of Sec. 18, T. 10, R. 8 E. Here are some ten 

 teet of very regularly bedded, yellow, sandy Hmestone, the layei-s below heavj-, abovo 

 turn and shaly, with fine large impressions of LHcellocephalus 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 : 



liOWEK MAGHaSIAU. 



JTecL 



1. QrsBsy slope, -wiQiont exposure .y 



2. Coarse, crumbling, brownish sandstone o 



3. Slope without exposures , -.n 



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



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

 sand grains , « 



MENDOTA AND POTSDAM. 



5. Coarse, brownish sandstone, in perpendicular ledges jg 



6. Slope without exposure or 



7. Friable, non-calcareous hght-colored sandstone j,5 



8. Sand-covered slope without exposmre ua 



Height above road at foot 1,05 



EiG. 40. 

 SeAion 2S Section 25 „^-S2-___ 



5=^=^^^ S.W. OirteOim at JecUm NSTE. yy^ 



Section of Kinoslst's Blupf, Lodi. 

 Horizontal scale 1,OJO feet to 1 incli. Vertical scale aOT feet to ] inch. Figures indicate altitudes 



in feet above Lake Michigan. 



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

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

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

 valley, tie accurate determination of whose stratigraphioal position meets with some 

 difficulties, and because of its somewhat pecuhar characters as compared with tiie 

 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 tlie sama 

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

 range of Merrimao. 



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

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



