BUILDING STONES. 153 



Quartzytes.] 



This stone is destined to be extensively used in the southwestern part 

 of the state, and in the states still further southwest, notwithstanding its 

 refractory nature, because of its accessibility, and the absence ot all other 

 kinds of building-stone, and at the same time it seems to be one of the 

 most promising formations for flagstone in the state, though it has not been 

 employed for that purpose. Similar quartzytes are found in the northern 

 part of the state. 



There is another silicious rock, perhaps deserving the name of quartzy te, 

 of a very different color and belonging in a very much later geological 

 period, which is seen at several points in the banks of the Minnesota river 

 between New Ulm and Mankato. It has supplied some very good building 

 material, and will also furnish flagstone. The layers are about four inches 

 in thickness as they appear after long weathering and are tough and firm. 

 They are associated with alternating layers of friable sandstone which aid 

 in their extraction. These beds are sometimes so coarse as to warrant 

 their being designated a conglomerate. The whole rock is light colored, or 

 sometimes rusty, and horizontally stratified. As a building material it is 

 very desirable, but the toughness and hardness of the texture, and the 

 thinness of the beds, make it more suitable for flagging than for building. 

 These beds are exposed on the N.E. J Sec. 16, Courtland, Nicollet county, 

 rising 35 or 40 feet above the river, favorably situated for working. Some 

 of the layers reach a thickness of six feet when they are wrought, this effect 

 arising from the union and cementation of several of the thin layers at some 

 depth within the quarry, a phenomenon which is common to all formations. 



Microscopic characters of No. 12. The quartz is in rounded grains from one-tenth"""- to 

 one mm. j n diameter. They all have the optical characters of crystallization. They are generally 

 not 'in immediate contact, but are separated by the cementing substance. They contain many 

 impurities which seldom have an evident crystalline form. There are also other large grains 

 which are now nearly or quite opaque from decay. These seem to have been originally some other 

 mineral, perhaps feldspar. Some of these are red in reflected light, and they give the color to the 

 rock, but when reduced in size they seem to be scatteringly disseminated even through the quartz 

 grains, where they do not appear red, but somewhat yellowish, and semi-transparent. The cement- 

 ing substance is composed partly of this red amorphous altered mineral. Figures 3 and 4 on plate U , 

 show this rock magnified forty diameters, the latter in polarized light. 



3. DOLOMITES. 



Under the term dolomite are embraced here only those magnesian 

 limestones that show, on analysis, at least as much as forty per cent, ot 

 carbonate of magnesia.* It will be seen by the table that, so far as they 



*Dolomite is a compound of carbonate of magnesia and carbonate of lime, the lime being 54.4 per cent, and the 

 magnesia 45.6. 



