BUILDING STONES. 169 



Dolomitic limeslones.] 



blue color within, but is faded to a drab to a considerable depth depending on the exposure, while 

 the immediate surface is generally a dirty buff. The grain is close, except for cavities resulting 

 from absorbed fossils. The pieces into which the stone weathers are brittle and somewhat sono- 

 rous. Contains abundant specimens of Orthis tricenaria, and Strophomena Minnesotensis, as well 

 as occasionally Murchisonia, Leperditiu and Edmondia. The fossils, however, are apt to be in 

 the form of casts and impressions. Thickness about eight feet. 



2. Similar to the last, but gradually becoming more impure with shale, the fossils being 

 gathered more into sheets or layers, making mere calcareous belts. Two feet. 



3. Green shale, calcareous, weathering blue, with but few fossils. Occasionally is found a 

 large specimen of Endoceras magniventrum, H. in this shale, the form only being preserved, sur- 

 rounded by a thin black film of bituminous matter. Four feet, eight inches. 



4. The last passes gradually into a calcareous shale resembling the well known building 

 rock of this place, in which still there are few distinguishable fossils. This stone is sometimes 

 used, like No. 1 above, for rough walls, or in protected positions. It is markedly set off from the 

 rock below by a projecting shoulder formed by the upper portion of No. 5. Two feet, four inches. 



5. Blue building-stone layers (No. 27 of the general table), used extensively at Minneapolis 

 and St. Paul. This stone is rather too argillaceous to be a reliable building material, yet it is 

 extensively used. The shale is intimately disseminated through the calcareous layers, without 

 showing regular lamination, yet causes a mottled, or blotched color over the surfaces when cut or 

 broken. The darker spots are shaly ; the lighter ones, which constitute the most of the rock, are 

 more purely calcareous. The color of the whole is bluish gray, which gives it the appearance of 

 strength and durability, when placed in a structure. The fossil remains in this number are apt to 

 be comminuted so as to be wholly undistinguishable, yet sometimes large pieces of Endoceras 

 magniventrum, II. are found in the layers. Rarely also on separating the layers in quarrying, a 

 rock surface is disclosed that is eminently fossiliferous with forms of BhynchoneUa, Orthis, and 

 other genera of brachiopods and mcrusting corals. This is the principal and most constant mem- 

 ber of the Lower Trenton. Thickness thirteen feet. 



6. Dolomitic limestone (No. 24 of the general table), somewhat vesicular, and of a dirty drab 

 color, less affected by shaly interlaminations than the last, in heavy beds that furnish a good 

 building material. This stone is used indiscriminately with the last in all places, but is evidently 

 a more valuable stone. Two feet. 



7. Blue shale, parting ehonchoidally under the weather, lying on the St. Peter sandstone. 

 Three feet. Total, thirty-five feet. 



These dolomitic layers from the Trenton (No. 22) are more durable 

 than the regular building-stone. The upper dolomitic layers do not appear 

 in the quarries near the falls, but they are seen in the quarries near the 

 university, and in those on the west side of the river at some distance be- 

 low the falls. The dip of the formation, and the erosions of the past, have 

 destroyed them at and above the falls of St. Anthony. This rock gener- 

 ally is rejected by builders, and is confounded with the worthless shale 

 (Nos. 3 and 4 of the section above) that separates it from the regular build- 

 ing stone layers. The older portion of the state university contains a large 

 amount of this stone, and its greater durability than that of the regular 

 building-stone can there be seen. The lower dolomitic stone from the 

 Trenton (No. 6 of the above section) is found in all the quarries. It is 

 generally not distinguished from the other building-stone layers, though 



