170 THE GEOLOGY OF MINNESOTA. 



[Limestones. 



sometimes the quarrymen have noted it under some fanciful description.* 

 Tt is represented by No. 24 in the general table. 



The other stone which in the general table is placed with the dolomi- 

 tic limestones (No. 25) is hardly worthy of being ranked with the building 

 stones of the state, both because it is a poor material for construction and 

 because it is not much employed. It contains 39 per cent, of insoluble 

 matter, mainly silica or silicate of alumina hydrated. The strata consti- 

 tute an earthy, or "argillo-magnesian" limerock, which contains much silica, 

 and vary in composition considerably from layer to layer. The layers are 

 usually thin, not exceeding five or six inches. It is fit for rough walls, if 

 the limy layers are carefully selected and the shaly ones rejected. This 

 stone is exclusively used at Lake City in Wabasha county, and at Hokah in 

 Houston county, although at each place the dolomites of the St. Lawrence 

 formation exist in the same bluffs in immediate superposition. At Lake 

 City, however, some importation has been made of the Berea sandstone from 

 Ohio (No. 41). Stone of the same kind is found, wherever, in the .state, that 

 geological horizon is seen in outcrop, and hence in most of the bluffs in 

 Winona, Houston and Wabasha counties, and the eastern portions of Good- 

 hue and Fillmore counties. It is near the top of the St. Croix sandstone, 

 where it begins to fade into the St. Lawrence limestone. 



5. LIMESTONES. 



The limestones that are used for building-stone in the state, are con- 

 fined to the Lower Silurian. None of them are strictly pure carbonate of 

 lime. The purest that has been observed is the rock formerly quarried 

 near Fountain in Fillmore county (No. 26), containing over eighty-six per 

 cent, of carbonate of lime, the most of the rest being insoluble matter, and 

 less than a half of one per cent, carbonate of magnesia. The rocks of this 

 group also vary to one containing nearly sixteen per cent, of carbonate of 

 magnesia, the same also having over twenty-five per cent, of insoluble mat- 

 ter. The last is from Clinton Falls in Steele county (No. 29), and pertains 

 to the Hudson River formation. 



The Trenton limestone as quarried near Fountain, is found in the bot- 

 tom of the formation, within ten feet of the St. Peter sandstone, in hori- 



*In some cases even styling it granite. 



