270 GEOLOGY OF EASTERN WISCONSIN. 



buff On the interior, the rock often has a greenish blue or gray 

 cast. Some of the thinner beds and shaly layers are variegated with 

 red and purple. From the ease with which the sandstone below is 

 eroded, the lower portion of the formation is often left projecting in 

 inural cliffs, or forming a' protecting crown for some isolated hill, 

 which oM^es its existence to such defensive covering. The strata dip 

 to the eastward, and are soon lost beneath the later formations, by 

 penetrating which the formation may be reached at continually in- 

 creasing depths, as we go eastward. 



The floor of the formation, so far as has been ascertained, is essen- 

 tially plane, but the ujqier surface is highly undulating or billowy, 

 for the latter term very accurately pictures to the mind its remarka- 

 ble nature. The billows of this petrous sea vary in height, from a 

 gentle swell to elliptical domes rising one hundred feet above their 

 bases, while their length ranges from a few rods to a quarter of a mile 

 or more, and their width, from one-third to one-half the length. The 

 regularity of outline here indicated is a frequent and typical, but not uni- 

 versal, fact. The slope of the sides varies from 30° downwards. The 

 axes of these domes lie in an easterly and westerly direction, much 

 more commonly than otherwise. 



Fig. 26. 



East and West Section nbak Eipon. 

 1. Lower Magneeian limestone. 3. St. Potei-B sandstone. 3. Trenton Umeutouo. 



The superficial strata of these rock-billows dip in every directum 

 from the center, most rapidly at the sides, and less so at the extremi- 

 ties; or, in other words, they are essentially concentric with the sur 

 face. 



These statements are made with reference to the original condition 

 of the mounds before erosion. There are satisfactory evidences that 

 during the deposit of the St. Peters sandstone upon this unequal sur- 

 face, the exterior of these prominences was somewhat eroded, and in 

 the removal of the latter formation by the elements and the drift 

 forces, resulting in their present exposure, they were still further acted 

 upon. 



The eastern and northeastern extremities suffered considerable abra- 

 sion from the latter cause. But neither of these agencies modified, 

 except superficially, the form of these prominences, while they served 

 to demonstrate more satisfactorily the quaquuversal character of the 

 dip. 



