THE LOWER SILURIAN ROCKS''. 



589 



The wostern face of the bluff is precipitous in its upper portion for over 100 feet. At 

 the top of the cliff is a rounded summit composed in part of glacial drift, but showing 

 in one place a few broken layers of hmestone (736), wliich are in "the proper position, and 

 have the proper characters for the " Buff " or Lower Trenton limestone. The cliff itself 

 is made up of iine-grained, hght-colored to nearly white, friable sandstone (735), which 

 is composed of angular and subangular quartz grains, and possesses a hard, vitrified 

 crust. In the uppermost paarts of the cliff the horizontal bedding is distinct, the layers 

 being quite thin; below, however, it is not plainly perceptible, whUst the whole has a 

 sort of vertically columnar appearance, due to jointing. On tlie upper part of the long 



Fig. 43. 



AU Cliff 

 US': 



_ Spit an d 2)riii 



120 Sundstane ^^ linexf^seA 



Bobdam. Sanidsime 



JdntiatA. &aU 



JH9 



Shotion op GmnALTEU Bluff. 



wooded slope below, are numerous very large sandstone masses, evidently fallen from 

 the cliff. At the lower edge of this slope the Mendota limesrtone is partly exposed, as 

 shown in Fig. 43, and below it the upper layers of the Potsdam, with intercalated cal- 

 careous bands. To the right and left of the line of section, lower non-calcareous sand- 

 stone layers are exposed, in low cliffs rising from the edge of the marsh. At the point 

 F, Fig. 42, on top of a bare hm, only a few rods from the sandstone chff, but a,t an 

 elevation of 40 feet above its base, is an outcrop of much disturbed Lower Magnesian 

 limestone. Numerous points on the smTOunding bluffs also show Hmestone at 



elevations above the base of the sandstone 

 ^i'*- ^- of the Gibralter cliff, proving the exist- 



ence of a very irregular upper surface to 

 the Lower Magnesian. 



For the district west of the Wisconsin 

 river, where both topography and strati- 

 graphy are so largely affected by the 

 quartzite ranges, it wiU be most suitable 

 to take up in order: the area south of the 

 the quartzite ranges; that west of the 

 ranges; that within them; and that north 

 of them. 



South of the quartzite ranges. Fig. 44, 

 which is a section from the top of the 

 quartzite range near the northwest cor- 

 ner of Sec. 2. T. 11, R. 8 E., Caledonia, 



Suction achosb the Vallet op the Wibcon- 

 siw IN Southeast Cai.bdonia. 



Vertical scalo, 350 fact to the Inch. Horizontal 

 scale, IJi miles to Oie inch. 



to the top of a bluff in Dekorra, serves to give an idea of the structure of this part of 

 the Wisconsin valley. 



On the flanks of the quartzite in western Caledonia, the Potsdam sandstone nses to 

 altitudes apparently in the horizon of the Lower Magnesian, having then a shght ap- 



