624 ^V. H. TWENHOFEL AND F. T. THWAITES 



units of decidedly different lithology which are separated by a 

 quite generally well-exposed greensand conglomerate. The strata, 

 which in this article are referred to the Franconia formation, are 

 included between the top of the Dresbach and the base of this 

 greensand conglomerate. 



The Franconia strata cap divides over considerable portions of 

 the northern halves of the two quadrangles. These have a rolling, 

 more or less terraced, topography with mammillary knolls. Where 

 overlain by higher strata the Franconia makes a terraced slope on 

 which at least two terraces are almost everywhere present. 



Relations to the Dresbach formation. — There is apparent con- 

 formity between the Franconia and Dresbach formations; but the 

 fact that there are some differences in the sequence at the top of 

 the Dresbach and the presence at the top of that formation of what 

 may be small erosion channels suggest that the contact of the two 

 formations is one of disconformity, with the possibility that the 

 lower was eroded before the deposition of the upper. 



Subdivisions and their characteristics. — The greater portion of 

 the Franconia formation consists of fine-grained quartz and glau- 

 conitic sandstones. The clay content is limited to thin shale 

 partings. A few beds contain small percentages of dolomite and 

 near the base there is a sandy limestone. There are few beds in 

 the formation which do not contain some greensand. 



The thickness of the Franconia varies from 120 to 173 feet. 

 The thickest sections are in the northern halves of the two quad- 

 rangles, although a well drilled at Cashton, just oft' the southeast 

 corner of the Sparta quadrangle, indicates a thickness of about 

 170 feet. The thinnest section, 120 feet, was measured near the 

 center of the Sparta quadrangle. 



It is possible to divide the Franconia formation into five 

 members, which in ascending sequence are: (i) the basal sandstone 

 and overlying calcareous layer, (2) the micaceous shale, (3) the 

 lower greensand, (4) the yellow sandstone, and (5) the upper green- 

 sand. These five members are continuous over the whole of the 

 two quadrangles and for considerable distances beyond their limits. 



I. The basal sandstone is roughly and irregularly bedded and is 

 everywhere cross-laminated. Colors vary from gray to brown with 



