550 GEOLOGY OF CENTRAL WISCONSIN. 



the formation is the rough-weathering seen on nearly all outcrops, in 

 some cases due to the concretionary structure of the rock. 



Chert occurs very abundantly throughout the Lower Magnesian, 

 and of three or four different kinds. In many places above the Mad- 

 ison sandstone there is to be seen a thin layer of greensand, and above 

 this a thinner one, one to ten inches in thickness, of a pure white, 

 oolitic, chert, which, on examination under the microscope, appears 

 to consist of egg-shaped aggregations of fine glassy quartz grains, em- 

 bedded in a still finer silicious matrix. This layer is very persistent, 

 in the country about Madison. A sample from the Madison quarries 

 yielded: silica, 98.01; alumina, 0.52; iron peroxide, 0.73; lime, 0.67; 

 magnesia, 0.21; water, 0.24= : 100.38. The same oolitic chert occurs 

 disseminated through the layers of limestone in the lower beds, to the 

 whole mass of which it often gives the appearance of an oolitic struc- 

 ture. The older writers on Wisconsin geology all speak of oolitic 

 limestone as characterizing the lowest portions of the Lower Magne- 

 sian; so far as my observation goes, the limestone is not oolitic itself, 

 but carries disseminated oolitic chert. Higher up, beginning usually 

 some 30 feet above the Madison sandstone, a more compact chert 

 comes in, increasing in quantity as the formation is traced upward. 

 This chert is either quite compact and flinty, occurring in irregular lay- 

 ers or nodules, or is more or less cavernous, the cavities being lined 

 with drusy quartz. Associated with the greyish-white beds of the 

 least silicious limestone, is a chert occurring in well-marked layers 

 and rows of nodules, which, in external shape and soft silicious coat- 

 ing, resemble closely the flints of the Chalk, whilst within they are 

 often beautifully banded and jasper-like. Still higher in the forma- 

 tion, about 100 feet above its base, the thin layers of limestone are 

 often replaced bodily, for considerable thicknesses, by a compact iron- 

 stained chert. 



Black dendritic markings are very common in the Lower Mag- 

 nesian, but occur in the greatest abundance and beauty in those layers 

 that are fine-granular and buff-colored, and not more than 30 to 40 

 feet above the upper surface of the Madison sandstone. The mineral 

 causing these markings is supposed to be the black oxide of man- 

 ganese. 



In stratigraphical arrangement, the Lower Magnesian appears 

 to show but little persistent regularity. An attempt to make out a 

 thoroughly reliable and detailed scheme of the stratigraphy of this 

 formation, meets with two considerable difficulties. The first of these 

 lies in the fact that, although exposures are very numerous, it is only 

 rarely that any considerable thickness can be seen at one' place; so 



