^4 



-.___, 



PRELIMINARY CHAPTER. ' 17 



the high bluffs it often presents this appearance. The ele- 

 vation is capped by the castellated Xiagara ; then conies a 

 gentle grass-covered slope, succeeded by the rough out- 

 lined, underlying Galena limestone, with numerous springs 

 flowing out near the base of the shales. This group is the 

 upper division of the Lower Silurian, the Xiagara being 

 the lower division of the Upper Silurian. 



The (i(dc)ui Lhiifxtone. This limestone, in lithological 



o 



character and general appearance, closely resembles the 

 Xiagara. It presents the same bold, castellated appearance. 

 The color is more uniform, being generally a light-creamy, 

 warm color, with shades of ashy-yellow and dirty-white. 

 The structure is more homogeneous and uniform, having 

 generally a sort of crystaline or sub-crystaline appearance, 

 except Avhen the lower beds sometimes assume, in their 

 passage into the underlying Blue, the characteristics of the 

 latter. The upper beds sometimes have a crumbling sandy 

 nature. The stone is massive, thick-bedded, solid, and be- 

 comes more enduring as it seasons. Its rich warm color 

 and enduring ilature make it a desirable material for heavy 

 masonry. 



The characteristic coral, the Receptaeultfes sulcata, "the 

 sun-flower coral" or "lead fossil"' of the miners, or "honey 

 comb" of the common quarryinen, is known to almost 

 every one. 



This is the famous "lead bearing" limestone of the Ga- 

 lena lead basin. It is heavily developed over the whole 

 lead region or lead basin of the Xorth-west, a basin occu- 

 pying an area, according to Professor WHITNEY, of about 

 4.000 square miles, and comprising portions of the States 

 of Iowa. Wisconsin and Illinois. It reaches a maximum 

 thickness of two hundred and fifty or three hundred feet. 

 It is unnecessary at this time to speak of its mineral trea- 

 sures in the shape of the rich deposits of lead ore found so 

 abundantly in its caverns, crevices, and decayed and super- 

 incumbent clay>. as in the first volume of these reports, 



