CHAZY GROUP. 37 



calcareous beds, sometimes difficult to distinguish from the Calciferous layers, fol- 

 lowed by beds of argillo-calcareous composition. 



§ 73. In 1852 David Dale Owen called it the "St. Peter's Sandstone," after 

 the river of that name, now sometimes called the Minnesota River, where it is usu- 

 ally made up of grains of limpid and colorless quartz, remarkable for whiteness. It 

 occupies part of the slope between the first and second terrace at Prairie du Chien, 

 forms the base of the bluffs at the St. Peter's, and the lower nineteen feet 

 at the Falls of St. Anthony. It rests upon the billowy surface of the Calcif- 

 erous, fills up the depressions, and is followed conformably by the Trenton. In 

 the lower part there is some shaly material and conglomerate matter washed from 

 the Calciferous and older rocks, but above this it is a remarkably uuiform, white or 

 yellow, friable quartzose sandstone, substantially free from silt and calcareous or 

 ferruginous cement. There are oblique and discordant lines of stratification, sup- 

 posed to be due to the shifting of the waves during deposition, and near the upper 

 surface there is more or less argillaceous material. In some localities it is tinged 

 yellow or red by the oxides of iron, and cemented in streaks, and weathers irregu- 

 larly. The outliers and standing rocks are brightly colored, and are called pictured 

 or painted rocks. The thickness will exceed 200 feet where filling a depression in 

 the Calciferous ; but the average thickness does not exceed 100 feet. Occasion- 

 ally ripple-marks, fucoidal impressions, and tubes of Scolithus occur in the harder 

 layers, but the only fossil yet described from this region is Lingulepis morsei. The 

 absence of fossils is due to want of preservation. 



§ 74. Prof. T. C. Chamberlin says the constituent grains of sand in this Group 

 are derived in the main from granitoid and schistose rocks, which are composed of 

 particles of quartz intermixed with a variety of softer and more decomposable crys- 

 talline minerals. In the metamorphism the quartz was usually last in crystalliza- 

 tion, and occupied the angular interstitial spaces between the crystals that had al- 

 ready taken shape, and hence while crystalline in internal structure it molded itself 

 about the crystals of the previously formed minerals. It was thus angular, but not 

 in its own crystalline form. Upon decomposition the associated minerals were mainly 

 reduced to earths and clays, while the undecomposable quartz remained in angular 

 grains. By the action of streams in carrying these down to the sea, and by the agency 

 of the waves in distributing them, the grains were sifted, assorted, rolled, rounded, 

 and finally deposited in the forms in which we now find them. The majority are 

 worn into somewhat spherical grains; others less acted upon remain quite angular. 

 The angularity, however, is not what is characteristic of freely forming quartz 

 crystals, but is due to the circumstances under which it was formed. In the orig- 

 inal crystalline rock occasional cracks and cavities occurred filled with secondary 

 quartz, which in such a situation assumed its own crystalline form ; and in the 

 sandstone itself secondary crystals might have been formed after deposition, just as 

 they have been in adjacent limestone-beds where their secondary origin is un- 

 questionable, and the degradation of the rock inclosing these would furnish points 

 and fragments of true crystals of quartz, which might not be so far worn as to lose 

 their characteristic form. 



§ 75. It occupies a narrow area fringing the Calciferous, or exposed in river 

 banks, stretching in an irregular course from the Lower Menominee River on the 

 north-eastern border of Wisconsin to the mouth of the Wisconsin River. It occurs 



4 



