154 THE GEOLOGY OF MINNESOTA. 



[Dolomite*. 



are employed as material for construction, the dolomites are confined to the 

 St. Lawrence formation, and at the same time that none of them reach the 

 percentage of magnesia required for pure dolomite. Besides the analyses 

 that have been made by the survey, Dr. Norwood's may be referred to, exhib- 

 iting the same fact, viz: 



Carb. lime. Garb. mag. 



From the shore of lake Pepin, 52. 42.2 



From lake St. Croix below Stillwater, 48.3 36.8 



From Gray Cloud island, a short distance above Hastings, 51.4 40.7 



From thirty miles below lake Pepin, 29.7 9.7 



It seems not only that the formation varies slightly from place to place, 

 in respect to the per cent, of magnesia, but also from layer to layer within 

 itself, since from the same quarry (as at Stillwater) the compact, even- 

 grained beds which are most highly prized for building, containing over 

 forty per cent, of carbonate of magnesia, alternate successively with 

 vesicular and irregular strata which contain somewhat over thirty-seven 

 per cent. The texture, however, does not always vary with the per cent, of 

 carbonate of magnesia in the same direction ; at Lanesboro the even-bedded 

 and compact rock contains between twenty-eight and twenty-nine per cent, 

 carbonate of magnesia, while the vesicular beds show forty -two per cent. 

 The vesicular texture of the Lanesboro rock, however, is not like that of 

 the rock from Stillwater with which it is here compared, but more like the 

 finely vesicular texture of the rock from Frontenac. The vesicular rock at 

 Stillwater is irregularly porous, or cavernous, and has a darker color. 



The St. Lawrence formation is the limestone which is conspicuously 

 exposed in the bluffs of the St. Croix and Mississippi rivers from Stillwater 

 to the Iowa state boundary. It generally forms the tops of the bluffs, and 

 causes the precipitous portions, the lower portions being made up of fallen 

 debris, hiding the underlying sandstones. It is not only seen abundantly 

 along these streams, but also along the bluffs of all the streams that flow 

 into the Mississippi from the west between Hastings and Brownsville. Of 

 the limestones of the state it affords more exposure, and is more generally 

 employed for construction, than any other. Throughout its whole extent in 

 Minnesota it furnishes a very excellent material for building indeed one 

 of the best, considered in all respects, to be found in the United States. Not 

 only does it furnish the dolomites (Nos. 13, 14, 15 and 16), but also many of 



