338 HALL AND SAUDESON — PALEOZOIC FORMATIONS OF MINNESOTA. 



Grand Rapids and Stevens Point,* On the southern borders, however, 

 the rim of the basin lies beneath other and subsequent formations. The 

 deepest known portion of this basin, is at Minneapolis, where granitic 

 rocks have been struck at 2,150 feet below the surface. The slope of its 

 bottom upward from this greatest depth is somewhat rapid toward the 

 northeast, where Keweenawan diabases appear at the surface within 35 

 miles, and the northwest, where granite quarries lie within 50 miles in 

 an air-line. Toward the south and southeast, however, the slope is more 

 gradual, as granitic rocks have been reached at La Crosse about 500 feet 

 below the Mississippi river.f 



Throughout the entire thickness of this sandstone, which at Minneap- 

 olis is nearly 1,550 feet J and at La Crosse 375 feet, are shown the ordinary 

 structural variations of a great sandstone formation. In places a heavy 

 conglomeratic character is observed; again a decidedly shaly condition 

 prevails. While everywhere a stratified condition is seen, in some places 

 this is much more marked than in others. It varies directly with the 

 variation from the sandy to the shaly condition of this rock, being most 

 complete with the latter, and is brought out beautifully when the rock 

 is subjected to erosion or weathering. With the filling of this basin and 

 the more rapid accumulation of sediments in its deepest portion a very 

 level floor was formed at a quite uniform depth below the sea level, on 

 which were laid down the dolomites and dolomitic shales of the great 

 Lower Magnesian series of Owen, the Saint Lawrence, Magnesian and 

 Shakopee of Winchell and Upham, with their interbedded sandstones. 



Lithologic Character*. — The conglomeratic, arenaceous, calcareous and 

 shaly phases of this formation have already been pointed out. In every 

 locality where its rocks have been observed a friable condition is con- 

 spicuous. Yet at Hokah, Dresbach, Dakota, Stockton and one or two 

 other places there is sufficient coherence or cementation to "encourage 

 quarrying; and, favorably for this business, the rock hardens on expo- 

 sure. Occasionally this coherence is secured b}^ the infiltration of a 

 cement of silica or through the compacting and partial alteration of the 

 rock itself, as at Dresbach and Dakota, but more usually through the 

 infiltration of calcium carbonate from the overlying dolomites and the 

 cementing together by it of the quartz grains. This condition is not so 

 common in these rocks as in those of one or two beds above them and 

 associated with the dolomites. It is not necessary here to give the anat- 



*See General Geological Map of Wisconsin, 1881. 



f From tin' records of the city engineer's office, La Crosse, through the courtesy of John James, 

 Esq. 



I Bull. Minn. Acad. Nat. Sci., vol. iii, no. 1, L889, pp. 125-143. 'lie- classification there used is 

 essentially Warren Upham's as given in the manuscript cited (page 335). In this paper forma- 

 tions 11, 12, 13 and 14 (see pp. 134, 135 of the Bulletin named) arc considered as one. and designated 

 Potsdam. 



