282 GEOLOGY OF EASTERN WISCONSIN. 



foot thick downwards. The rock weathers to a granular, sandy surface, much pitted 

 from unequal resistance to the elements, and showing lines of deposition, o.l feet. 



2. A group of beds very similar to the above, but characterized by a much greater 

 abundance of crystalized quartz, lining the numerous small cavities, or forming small 

 clusters. There is no distinct line of demarkation between these and the above layers. 

 11.7 feet. 



3. Below the above lies a thick bed of impure conglomeritic dolomite, the component 

 fragments having an almost flinty hardness and fracture. It presents the appearance 

 of having been formed of fragments of silicious dolomite imbedded in a matrix of dolo- 

 rnitic sand and mud, which afterwards cemented, and in a measure, coalesced. On 

 weathering, the constituents are brought out conspicuously. The more compact frag- 

 ments seem to contain a considerable percentage of silica disseminated through them, 

 while segregations of quartz, much oftener in the crystalline than the nodular form, are 

 very numerous, and, standing out upon the weathered surface, give it a very rough as- 

 pect. This layer by its hardness offers great resistance to erosion from the volume of 

 water pouring over it, but when undermined by the removal of the softer rock below, 

 it falls in huge masses, sometimes 20 feet in maximum diameter, which lie in the chan- 

 nel for ages before complete removal. The prevailing color of the rock is dark gray, 

 mottled by the-whitp quartz, and the variously colored fragments of which it is composed. 

 The thickness of the bed is varying, that measured as an average being 4 . 7 feet. 



4. Underlying the above is a very hard, impure dolomite of a dark gray color, mottled 

 with lighter hues, and of more uniform texture than that above, but still not homogene- 

 ous, while some portions are studded with small geodes and clusters of quartz crystals. 

 It is not subdivided into regular beds, but is much fissured vertically. 9.7 feet. 



5. Still lower lies a stratum of less hardness, much finer and more uniform crystalline 

 grain, and more distinctly laminated, so as to present a horizontally banded appearance. 

 It has a bluish gray cast on the fresh, somewhat conchoidal fracture, distantly stained 

 with dark purplish brown iron spots, but weathers to a dirty gray. The quartz segrega,- 

 tions descend from above into the upper layers of this. 4.7 feet. 



An analysis of this stratum by Mr. G. Bode gave: 



Carbonate of lime 49.414 



Carbonate of magnesia 39.784 



Silica 7 . 638 



Alumina 1 .473 



Oxide of iron 1 .691 



100.000 



6. Below this follows a bed of green and purple shale and argillo-arenaceous dolomite, 

 having the following composition: 



Carbonate of lime 29.370 



Carbonate of magnesia 18.860 



Silica 37.798 



Alumina 9.621 



Oxide of iron 4. 351 



100.000 



This shale very closely resembles that of the Eureka section, and probably belongs to 

 the same horizon. It is to the softness of this shale and the ease with which it is eroded, 

 that the falls are due. 2 feet. 



