V 



190 THE GEOLOGY OF MINNESOTA. 



[General conclusions. 



ered in the discussion or selection of a building-stone. The more nearly 

 these three are combined in favorable degrees in the same stone, the more 

 valuable that stone becomes. 



Previous to the opening of a quarry the rock in its natural beds should be 

 carefully examined, and a full and scientific description, including a compari- 

 son of the different layers, ought to be made of the appearance of the rock in 

 the hands of nature, as testimony to its weathering power. The enduring 

 formations are the sources of our best building-stone, and the individual 

 layers will be found conspicuous, or hid by the decay of their own forms, in 

 proportion as they have been able to withstand the alternations of heat and 

 cold and of moisture and dryness. There they have been subjected to these 

 vicissitudes for thousands of years. 



Permanence of color, and the presence of pyrite, may be determined best 

 by the appearance of the natural outcrop. Before demolition by the weather 

 a rock assumes different colors. Whatever may be the color of a stone fresh 

 from the quarry, it can not be permanent. Most of our limestones show 

 three different colors, according to the degree of exposure and the ease ot 

 access within for air and water. For instance, the Galena limestone, in the 

 southern part of the state, as seen along the river bluffs, is of a dark buff or 

 rusty -buff color on the long- weathered surface. This color pervades that 

 portion of the stone that is undergoing rapid disintegration in its natural 

 position, and is generally not more than half an inch in depth. Under that~ 

 is the light buff color seen in most of the stone quarried from that forma- 

 tion, and it pervades the stone to a great depth. Where the texture is 

 open this color has apparently gone through all the beds, but it is an 

 acquired color produced on the rock by atmospheric causes. The same 

 effect is produced on all magnesian limestones, the depth of the change 

 depending largely on the percentage of argillaceous impurities the rock 

 may contain. Deep within the rock may finally be seen the natural color, 

 where exposure has not yet produced any change. It is blue, and at first is 

 seen only in the centre of the layers, surrounded by a layer of the light 

 color. These different colors can be seen in the Trenton at St. Paul and 

 Minneapolis; but owing to the close and clayey nature of this formation, 

 the blue color is better preserved, and is that seen in the stone now most 

 used. Formerly the stone used was of a dirty-drab or yellowish color, very 



