222 GEOLOGY OF EASTERN WISCONSIN. 



magnesian limestone, and were apparently derived from the Silurian 

 formations prevalent in the region. In some instances this is put be- 

 yond question by the occurrence of Niagara fossils in them. The sur- 

 faces and angles of those fragments are worn, but not to such an ex- 

 tent, usually, as to give them a symmetrical rounded form, and it is 

 especially to be noted that the surface is scratched and subtranslucent 

 instead of having an opaque, somewhat granular surface, such as arises 

 from the wearing and solvent action of water. Occasionally a bowl- 

 der of considerable dimensions occurs within the formation, but there 

 is a very marked difference between this formation and the Bowlder 

 Clay in that respect. This fact becomes a very conspicuous one in 

 examining adjacent areas, occupied by the two formations. The sur- 

 face of the Red Clay is comparatively free from bowlders, while they 

 occur in the very greatest abundance on the surface of the Bowlder 

 Clay. It may be here remarked that there is no such abundance of 

 bowlders on the surface of the Red Clay district as to suggest any 

 other origin than that of wash from the formation itself ^. The clay 

 which constitutes the chief element is highly calcareous in character. 

 It contains, also, a considerable portion of finely comminuted quartz- 

 ose material, in addition to true aluminous clay, so that while the 

 whole is quite compact and impervious, it yet has not that extreme 

 toughness and adhesiveness that is possessed by the typical aluminous 

 clays. 



A considerable ingredient of magnetite, in the form of minute 

 grains, is present. A magnet drawn through a handful of the pulver- 

 ized clay usually brings forth more or less of this mineral. Along 

 the lake shore, where the clay is washed away by wave action, the 

 magnetite is left as a deposit of black sand, mingled with the quartz 

 sand of the beach, whence it may be collected in large quantities. 

 The lower part of the deposit is usually very highly laminate, and pas- 

 ses, by alternations and gradations, into the subjacent beach deposit. 

 The main mass of the formation presents little indication of the lines 

 of deposit, but upon close inspection, minute lamination may be ob- 

 served. The relation of this to the imbedded rock is interesting. As 

 the laminae approach the fragments, the greater portion curve down- 

 ward and pass beneath it, while a portion curve over it. It would 

 appear from this fact that the fragments were dropped upon the sur- 

 face of a yielding clay mass, and covered by subsequent deposit. Near 

 the southern extremity of this formation, it has, along the lake shore, 

 a thickness of from 15 to 20 feet. At "Whitefish Bay, above Milwau- 

 kee, it is from 20 to 30 feet thick, near Ulao, 40 feet, and in the vicin- 



' Compare Geological Survey of Ohio, Vol. II, Part 1, page 4. 



