164 THE GEOLOGY OF MINNESOTA. 



[Dolomitic limestones. 



to that from the St. Lawrence, in outward characters, a circumstance which 

 explains the reference of its outcrops to the same horizon as the St. Law- 

 rence, by Dr. Owen and othei's. In chemical constitution, however, it is 

 found to be rather more silicious, having more frequent cherty beds or 

 nodules, and distinctly arenaceous portions. Toward the Mississippi valley, 

 however, this limestone becomes thinner, and its line of outcrop is found 

 inconspicuously running along the valleys at same distance back from the 

 tops of the bluffs, and often invisible over many square miles. It is very 

 seldom wrought for building stone in the counties bordering on the Miss- 

 issippi river, though its integrity even to the eastern portions of Winona 

 county, distinct from the St. Lawrence, has been fully established by care- 

 ful field examinations. The most eastern point where it is worked is at 

 Troy, in Winona county, where the Troy mills have been built from it. It 

 is seen there twenty-five feet thick, underlain by the Jordan sandstone, 

 whereas, at points in the Minnnesota valley it exhibits a thickness of about 

 seventy feet. 



At Shakopee and Louisville, in Scott county, this rock is principally 

 employed for lime-burning. The same is true at Ottawa, in Le Sueur county. 

 At Kasota, in Le Sueur county, it is quarried extensively, and is shipped to 

 distant markets under the name Kasota stone. Several large quarries were 

 opened in this stone at St. Peter, and the stone from them was largely 

 used in the construction of the older portions of the insane asylum at that 

 place. Very extensive working of the same beds is carried on at Mankato. 



The rock is sub-crystalline, homogeneous, and rather compact than 

 vesicular. 



Microscopic characters of the dolomitic limestones. The dolomitic limestones of the St. Law- 

 rence formation (Nos. 17, 18 and 21) do not differ from the dolomites in microscopic characters. 

 In chemical composition the principal difference is the lower percentage of carbonate of magnesia 

 in the aggregate. But this does not seem to express itself in any perceptible manner in the 

 forms of the crytalline grains. Indeed, the percentage of carbonate of magnesia sometimes runs 

 very low in the double carbonate of lime and magnesia, and the change 'cannot be distinguished 

 under the microscope, because the crystals of dolomite and calcite have the same forms. Still, as 

 the mineral becomes pure calcite, very evident colored bands, due to a twinning on the face 

 J E are developed when the analyzer alone is used. This, however, has not been seen in any of 

 the sedimentary limestones of Minnesota. Figure 2, plate C, shows the manner in which the 

 impurities resulting from decay pervade the central portion of the individual crystalline grains in 

 the rock No. 17. The same change is perceptible in the rhombohedra of Nos. 18, 19 and 23. In 

 the last, however, the colored area is enlarged, and sometimes embraces uniformly the whole grain 

 Some of the grains also in No. 23 are evidently irregular detached parts of larger rhombodedra . 

 showing traces of cleavage in one direction. In general, the dolomites and dolomitic limestones 

 are nearly free from quartz granules ; the principal acquired impurity is ocher. The granules of 



