THE LOWER SILURIAN ROCKS". 



589 



The western 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 limestone (736), which are in the proper position, and 

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

 is made up of fine-grained, light-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 parts of the cliff the horizontal bedding is distinct, the layers 

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

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



FIG. 43. 



Soil <rnd Drill 



SECTION OF QranALTKu BLUFF. 



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

 the cliff. At the lower edge of this slope the Mendota limestone 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 hill, only a few rods from the sandstone cliff, but at an 

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

 limestone. Numerous points on the surrounding bluffs also show limestone at 



elevations above the base of the sandstone 

 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 will 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, 



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 rises to 



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



SDCTION ACROSS THE VALLEY OP HD WISCON- 

 SIN IK SOUTHEAST CALEDONIA. 



Vertical scale. 350 foet to the inch. Horizontal 

 scale, 154 miles to the inch. 



