BUILDING STONES. 155 



Dolomites.] 



those here classed as dolomitic limestones, which rank (some of them) higher 

 on the comparative scale than some of the dolomites. It shows also some- 

 times a percentage of insoluble matter as high as ten or eleven, which seems 

 to replace carbonate of magnesia rather than carbonate of lime. When, 

 however, the insoluble matter is largely aluminous it seems to replace also 

 carbonate of lime, and the comparative rank of the stone as a building 

 material is injured. 



An analysis of a sample from Sugar Loaf, Winona, largely used in the 

 State Normal School at that place, gave the following result. 



Insoluble (mainly quartz),' 24.21 



Ferric and aluminic oxides, 3.32 



Calcium sulphate, 4.32 



Calcium carbonate, 47.11 



Magnesium carbonate, - 20.67 



Total, 99.72 



In this case the high rate of the per cent, of silica was due partly to the 

 existence of silicious aggregations isolated from the mass of the rock, and 

 partly to the geodic cavities lined with fine quartz crystals. The bulk of the 

 rock probably would not show any higher rate than ten or eleven per cent. 



Microscopic character of the dolomites, Nos. 13, 14, and 16. No. 13 is seen in plate B, fig. 5, 

 No. 14 in plate B, fig. t>, and No. 16 in plate C, fig. 1. They are all magnified forty diameters. 

 The first (No. 13) is interspersed with natural cavities which the rock shows to the naked eye. 

 These seldom exceed a millimeter in diameter. In some places there are similar spots in the thin 

 section, which are now filled with a very fine-grained substance which has the same general color 

 as the rock itself, but which appars isotropic between crossed Nicols, and if highly magnified does 

 not exhibit any crystalline forms of microliths. Sometimes these isotropic spots have a dim con- 

 centric banding of light and dark, as if they were due to successive accretions from the surround- 

 ing rock. Sometimes between crossed Nicols they show a black cross which becomes dissipated 

 on rotation and returns again. The whole rock is somewhat stained with ochre, and shows very 

 rarely a small grain of quartz. The individual grains of dolomite are small, and do not often show 

 the two cleavage systems. 



No. 14 is a much more dense rock. Its individual grains are from one-fiftieth"""- to four- 

 fiftieths"""- in diameter. They are angular, but show no cleavage lines. They are flecked with 

 numerous impurities. When seen with a low magnifying power they polarize between crossed 

 Nicols in colors of blue and yellow. This rock also contains an occasional grain of quartz. 



No. 16 is very similar in all respects to No. 14, but is somewhat coarser, some of the larger 

 grains sometimes having a trace of the natural cleavage remaining, as well as the rhombic form of 

 the crystals. 



The dolomites here spoken of, and the dolomitic limestones from the 

 same formation (Nos. 17, 18 and 21), are of a buff color, varying to a light 

 drab, the latter appearing in the coarsely vesicular beds, as No. 18, from 

 Stillwater, and they have therefore a lively and cheerful expression in any 

 building. The rock is but slightly changed after many years of exposure 



