390 GEOLOGY OF EASTERN WISCONSIN. 



LOWER HELDERBERG LIMESTONE. 



Four miles northwest of tlie city of Milwaukee — Sec. 7, town of 

 "Wauwatosa — in the banks of Mud Creek, are two low exposures of a 

 shaly limestone, that differs in lithological character from both the 

 Niagara limestone, upon which it rests, and the Hamilton cement 

 stone, by which it is overlaid. The rock is a hard, brittle, light gray, 

 magnesian limestone, distinguished by numerous minute, angular 

 cavities, that give to it a very peculiar, porous structure. It is thin- 

 bedded and laminated, by virtue of which it splits very readily into 

 flags and thin plates, which are, for the most part, too brittle, and too 

 much subject to further splitting, to be serviceable as paving, but 

 which are considered valuable for Macadamizing. A transverse frac- 

 ture of some of the layers exhibits an alternation of gray and dark 

 colored laminse, peculiarly characteristic of this formation. 



The rock is a nearly pure dolomite, as shown by the following an- 

 alysis, by Mr. Bode: 



Carbonate of lime 54.569 



Carbonate of magnesia ' 43.410 



Silioa 1.494 



Alumina 0.211 



Oxide of iron 0.316 



100.000 



The two quarries are less than half a mile apart, and lie on a near- 

 ly east and west line. On this line, a little less than two miles to the 

 east, along the Milwaukee river, a short distance above the Washing- 

 ton street bridge, the Hamilton cement stone is found to rest upon a 

 dark brown, ferruginous rock, that, to casual observation, bears little 

 resemblance to that above described, but which, upon closer inspec- 

 tion, is found to possess, in a large degree, the same peculiar porous 

 and laminated structure, and to differ from it chiefly, in being a lit- 

 tle less shaly and much more ferruginous. Its chemical composi- 

 tion is shown by an analysis, by the above named chemist, to be as 

 follows : 



Carbonate of lime 54.693 



Carbonate of magnesia 41 .818 



Silica , 1.575 



AJumina 0.478 



Oxide of iron 1.436 



100.000 



