286 GEOLOGY OF EASTERN WISCONSIN. 



while in the immediate vicinity, observed depths of sandstone of fifty- 

 four feet, eighty-two feet, and one hundred feet have been noted. 

 This irregularity appears to be greatest from Dodge county north- 

 wards. In the southern part of the district, and, so far as the Artesian 

 wells enable us to judge, in the lake shore region, a greater degree of 

 uniformity prevails. In other words, the unevenness of the Lower 

 Magnesian surface seems to have been greatest ^ear its margin, or 

 near the shore line of the ocean at the time of its deposit; and as it 

 recedes to the eastward and southward, it becomes more uniform. 



Setting aside, for the moment, a large number of exceptional cases, 

 arising chiefly from the foregoing peculiarities, the rock may be de- 

 scribed essentially as it is found to be elsewhere, being composed of 

 well rounded, rather uniform, transparent, incoherent grains of quartz, 



forming a very friable rock. 

 Calcareous and argillaceous 

 matter are almost entirely ab- 

 sent. White, yellow and gray 

 are the most prevalent colors, 

 but red, brown, pink and 

 green are not uncommon. 

 Sometimes the rock is beauti- 

 fully variegated and in other 



SHOWING COLORATION OF ST. PETERS SANDSTONE AT ^aSCS " 1S Danued in a U1S- 



RlFON - jointed and irregular way, 



The shaded portion represents brown sandstone; the nrodnfMno- a vprv'intprpstincr 

 remaining portion is white, lined and flecked with pink. " 



and unique effect. This is 



imperfectly illustrated in the accompanying figures. In the upper 

 part of the formation, irregular concretions of iron ore occur, which, 

 on weathering, present a dark brown, glazed surface, which leads to 

 the popular impression that they are of volcanic or meteoric origin. 



In structure, the St. Peters sandstone is finely laminated, fre- 

 quently in oblique and cross lines, and shows at some points fine ex- 

 amples of ebb and flow structure. The bedding is usually obscure. 

 Vertical, oblique and irregular fissures frequently traverse the forma- 

 tion, cutting it into huge, irregular masses. 



The exceptions to these general characteristics arise chiefly from 

 the relation of this sandstone to the irregularities of the underlying 

 formation. Where it adjoins the sloping sides of the limestone 

 mounds, it has derived from them ingredients not possessed else- 

 where. More or less of calcareous matter would necessarily become 

 mingled with the sand during its deposit. The amount observed is 

 not so great as might be expected, and sandstone within a few feet of 



