BUILDING STONES. 177 



Sandstones.] 







Potsdam formation, near the horizon of its passage into the St. Croix. The 

 Hinckley rock is but slightly exposed, and has not been much wrought, but 

 on account of its evidently very superior qualities, it will be sought for more 

 for general building than heretofore, and hence it is probable that some of 

 its exposures farther clown the valley of the Grindstone river will be opened 

 for that purpose. The present quarry is situated on the left bank of the 

 Grindstone river where the railroad crosses it, the rock rising about eight 

 feet above the water. The railroad bridges along the dalles of the St. Louis 

 river have foundations of this stone, and some other railroad works have 

 been made with it. Tts color is light-yellow or sometimes with a pinkish 

 tint, and its grain is uniform and arenaceous. Its strength under pressure 

 is very great, amounting to 17,500 pounds per square inch when placed on 

 edge, and 19,000 pounds per square inch when on bed. 



Microscopic characters of Nos. 30 and 31. The largest of the individual quartz grains com- 

 posing this rock are about one-half millimeter in diameter, but in general they are about one- 

 tenth millimeter. They are rounded as by attrition among themselves, and their shape is sub- 

 angular. Their exterior is minutely roughened and in these depressions is deposited the cement 

 which furnishes the bond of the rock. Many of the grains also are interpenetrated by impurities 

 which are ferruginous and give them a yellowish tint, but for the most part the quartz is pure and 

 perfectly clear. Cavities in the quartz grains are sometimes in parallel lines. While the exte- 

 rior of the quartz grains is coated with a sprinkling of yellowish ochreous impurities, the cement- 

 ing bond is largely calcareous, but so meager that there are interstitial cavities bet ween the grains 

 of quartz. Figures 5 and 6, plate C. 



The sandrock No. 31, quarried near Fort Snelling, has occasionally a grain of feldspar min 

 gled witli the quartz, and some of the quartz contains small acicular crystal inclusions that 

 resemble apatite. Otherwise it is exactly like No. 30. It has, however, a little more copious and 

 darker-colored cement, with an occasional rounded grain of magnetite. 



Rock No. 31 has been used in the foundations of the railroad bridge at 

 Port Snelling, one of the piers of this bridge being constructed mainly of 

 this stone, and in the piers of the new highway bridge at the same place, 

 crossing the M ississippi river. The quarry is on an island near Fort Snelling, 

 so near the water level of the Minnesota river that it can be operated only 

 in the winter, when the blocks can be hauled away on the ice. The Port 

 Snelling quarry was opened in 1869, but was not used very much, after the 

 construction of the railroad bridge over the Minnesota, till 1878, when the 

 highway bridge over the Mississippi was constructed. The strata are homo- 

 geneous, horizontal and about two feet thick as quarried, the color being a 

 rusty yellow with some broad banding of lighter yellow. It is liable to 



inequalities in hardness and durability. Its color gives it an attractive 

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