272 GEOLOGY OF EASTERN WISCONSIN. 



spots, and in part to the presence of nodules of chert. This inequality of structure is 

 exaggerated by the effects of long weathering, giving the surface a very ragged aspect. 

 The exterior of the chert is usually white and rather soft, while the interior is dark or 

 reddish, hard, translucent and flint-like. The outcrop represents the upper portion of 

 the formation. 



Tin's limestone next appears to the north, within our district, on the Crawfish river, two 

 miles below Columbus, in a low exposure in the banks of the stream, similar to the 

 above. Three-quarters of a mile northeast from this, near the center of the S. E. ^ 

 sec. 19, town of Elba, a quarry exposes the following section of impure magnesian lime- 

 stone: 



1. A brecciated stratum, composed of small, compact, dark gray fragments between 

 which are numerous irregular spaces filled with white pulverulent material. Thickness, 

 2 feet. 



2. Beds more homogeneous than the above, yet porous in parts and compact in oth- 

 ers, containing nodules of chert. Thickness, 4-6 feet. 



3. A very highly brecciated layer, formed of compact, dark colored dolomitic frag- 

 ments, covered with black dendritic crystals, and full of small, irregular crystal lined cav- 

 ities. Thickness, 1.45 feet. 



4. A stratum of very uneven texture and composition, containing silicious nodules. 

 Thickness, 5. 7 feet. 



5. A single thick-bedded, porous, moderately soft dolomitic layer of uneven texture. 

 Thickness, 2.3 feet. 



6. A band of rather thin sheets of irregular arrangement, inclosing large, coarse, nod- 

 ular masses of breccia-like rock, hard, compact, cherty, red stained, some portions ap- 

 parently silicious, some nearly pure dolomite. Thickness, 4.35 feet. 



7. Thick, uniform bed of moderately hard, compact texture, the upper portion 

 marked with greenish silicious sand, iron stained. Thickness, 2. 75 feet. Total, 21 . 15 feet. 



North of this, in Dodge county, this formation shows an occasional limited exposure, 

 similar to the preceding. 



In Green Lake County it has already' been remarked that the Lower Magnesian 

 strata crown several of the prominent hills. These are the lower layers of the forma- 

 tion, and constitute the small patches lying west of the serrated edge of the main body 

 of the formation, as represented on the accompanying maps. 



The extensive prairies of Manchester and Dayton townships are underlaid by this 

 rock, while those to the eastward repose on the Trenton and Galena limestones. It is 

 in this county that we first find a clear demonstration of what constitutes the most re- 

 markable feature of the Lower Magnesian limestone, viz. : the undulatory nature of the 

 upper face of the formation already mentioned. North of Lake Maria, in the town of 

 Mackford, are several conical mounds, partially covered with earth, which rise nearly 

 to the level of the base of the Trenton limestone, which occurs a few rods to the east, 

 while in the town south, at least 23 feet of sandstone intervene, and in the town north, 

 at least 82 feet, and near Eipon, over 100 feet. So far as exposed, the strata forming the 

 mounds dip away from the center. But we need not linger upon so poor an exposure. 



In the S. E. qr. of Sec. 7 of the same town is a conical hill surmounted by a turret 

 of massive breccia, which needs to be noted in this connection, though there is nothing 

 except its elevated position to show that it indicates any irregularity in the surface of 

 the formation. But in the N. E. qr. of Sec. 15 of the same town, at the point where 

 the road crosses a small stream, there is a most satisfactory demonstration of this in the 

 rise of an arch of Lower Magnesian strata into actual contact tvith the Trenton lime- 

 stone, causing the thinning out of the St. Peters sandstone to absolute zero. This is il- 

 lustrated by the accompanying figure. 



The layers of the Lower Magnesian at tin's point, especially as seen down the stream 



