THE TOM AH AND SPARTA QUADRANGLES 615 



for Wisconsin, and one of Walcott's papers, in which is defined the 

 geologic section for western Wisconsin as worked out by Ulrich.^ 

 The latest published work relating to the areas is an abstract of 

 a paper read before the Geological Society of America at the 

 Albany meeting. This paper describes the rock terraces which are 

 such conspicuous features of the surface.^ 



DESCRIPTION OF THE GEOLOGIC SECTION 



Only rocks of sedimentary origin are exposed in the Tomah and 

 Sparta quadrangles (Fig. i). Of these there are two groups: an 

 older, consisting of poorly cemented sandstones and compact, firm 

 dolomites, both of Paleozoic age; and a younger, made up of uncon- 

 solidated materials of very late geologic time. With the latter 

 this article is not concerned. The strata of both groups are in 

 essentially undisturbed position. 



The Paleozoic strata belong to the Cambrian and Ordovician 

 systems.'' The exposed thickness aggregates between 700 and 

 800 feet, while wells which have been drilled to the base of the 

 sedimentary sequence show the presence of an additional thickness 

 of about 350 feet. These unexposed strata are of Cambrian age. 



CHARACTER AND ROCKS OF THE PRE-CAMBRIAN FLOOR 



The rocks composing the pre-Cambrian floor beneath the Pale- 

 ozoic sediments are known from deep wells and from exposures 

 some miles to the north. Drillings from a well in Tomah indicate 

 a gneiss of medium texture, composed of clear and milky quartz, 

 pink feldspar, and white mica. The nearest exposures to the north 

 are at Black River Falls, where the pre-Cambrian rocks consist of 

 granites, gneisses, diorites, schists, and iron formation. 



The exposures of the contact between the Paleozoic and pre- 

 Cambrian rocks and the wells in this and adjacent districts which 



' C. D. Walcott, "Cambrian Geology and Paleontology," SmUhsoman Misc. Coll., 

 LVII, No. 13 (1914), 354- 



^Lawrence Martin, "Rock Terraces of the Driftless Area of Wisconsin," Bull. 

 Geol. Soc. Amer., XXVIII (1917), 148-49. 



^ If the classification of Dr. E. O. Ulrich be followed, there would be three systems 

 represented: Cambrian, Ozarkian, and Ordovician. Fig. i shows the groupings of 

 the systems according to the general usage and also according to the proposals of 

 Dr. Ulrich. 



