THE GLACIAL DRIFT. 623 



little south of east. Devil's Lake lies in the north and south portion 

 of the gorge. At its northern end a hill of drift rises al)ruptlj from 

 the water to a height of 100 feet, falling on the further side as ab- 

 ruptly over 200 feet to the Baraboo river. A short distance beyond 

 the sonthern end of the lake a similar hill chokes the gorge from side 

 to side, rising 100 feet from the lake level, and on the eastern side 

 sinking rapidly until at its eastern end the bottom of the gorge is 

 full 150 feet below the lake. Through this hill a deep cutting is 

 made for the Chicago and Northwestern Railroad. The sides of the 

 cutting show no sign of stratification, but only a sandy tenacious clay 

 with numerous sci-atched pebbles and bowlders, the latter including 

 the usual kinds of crystalline rocks, but also a number of quartzite, 

 some of which are much smoothed and striated. The large drift cut- 

 ting near Baraboo shows something the same sort of material, which 

 is, however, much more sandy, and has traces of a crude stratification. 

 It is quite probable that till -like clays occur somewhat widely in the 

 region of the Kettle Range, but the rare cuttings make this conject- 

 ural only. In southeastern Adams county, in the region about Big- 

 Spring, quite a large area occurs in which the surface material is a 

 red tenacious clay. No cutting was seen in this clay, and its exact 

 relations and structure are doubtfiil. 



Stratified clays, often fine-laminated, are found in the valleys of 

 most of the streams in the southern part of the Central Wiscon- 

 sin district, where they are interstratified with fine gravel and 

 sand, and are often utilized for making brick. Such clays are founcf^ 

 at a number of places in the Catfish Valley, as, for instance, in the 

 Vicinity of Madison, at Oregon, at Stoughton, etc., at times yielding 

 a pure white or cream colored brick, at others, an ordinary red brick. 

 The following analysis is of one of the latter kind, from a pit in the 

 valley through which the Milwaukee & St. Paul railroad passes, on 

 the S. E. qr. of Sec. 17, T. 7, R. 9 E., about one mile west of the 

 University at Madison: silica, 75.80; alumina, 11.07; iron peroxide, 

 3.53; iron protoxide, 0.31; lime, 1.84; magnesia, 0.08; carbonic di- 

 oxide, 1.09; potassa, l.U; soda, 0.40; water, 1.54; hygroscopic moist- 

 ure, 2.16 = 99.56. 



These clays contain occasionally small pebbles of limestone which, 

 on being baked in the middle of the brick will subsequently " slack " 

 and cause it to burst open. The clays that produce the light or 

 cream-colored brick contain not uufrequently as much iron as the 

 ordinary red clays, but are very much more calcareous, resembling in 

 this regard the famous Milwaukee brick clay. 



The dififerent behaviours of these two classes of clay under heat is 



