188 THE GEOLOGY OF MINNESOTA. 



[General conclusions. 



It is evident therefore that the value of a building-stone depends both 

 on its chemical constitution and its physical structure. A rock so consti- 

 tuted that either of these is liable to rapid attack by the weather, is neces- 

 sarily a poor one for construction. 



A stone that absorbs moisture abundantly and rapidly is apt to be 

 injured by alternate freezing and thawing. Hence clayey constituents are 

 injurious. An argillaceous stone is generally compact, and often has no 

 pores visible to the eye. Such will disintegrate rapidly either by freez- 

 ing and thawing, or by corrosive vapors. 



A stone that is compactly and finely granular will exfoliate by freezing 

 and thawing more easily than one that is coarse-grained. 



A stone that is laminated in structure, so as to absorb moisture unequally, 

 will expand by heat and contract by cold unequally, and especially by 

 freezing and thawing. Such a stone will gradually separate into sheets 

 coincident with its laminated structure. 



A stone that has a granular texture, as contrasted with one that is crys- 

 talline or fibrous, will crumble sooner by frost and by chemical agents, 

 because of the easy dislodgment of the individual grains. 



A stone which has an open texture will serve as a lodging place for float- 

 ing particles of dust, and lichens and fungus growths will appear on its 

 surface. These give off by their decay organic acids which attack the car- 

 bonates, of which the rock may be largely composed, and unless removed 

 they will cause the decomposition of the stone to some depth, and its con- 

 version to a loose powder. A loose-textured stone, however, does not trans- 

 mit heat so rapidly as a dense one ; and in cold climates, especially if they 

 are also dry and free from dust, it would be a warmer building material as 

 well as perhaps more durable, than a dense stone. 



A doloinit ic limestone is more durable than a pure limestone. The lime- 

 stones of Minnesota are all of them somewhat magnesian, but the Trenton 

 is not sufficiently so to be distinctively denominated a magnesian limestone. 

 Nearly all the others are so highly magnesian as to reach occasionally 

 more than forty per cent, of carbonate of magnesia, when they have been 

 classed as dolomites in the accompanying table. 



A siliceous rock, other things being equal, is more durable than a lime- 

 stone; but the durability of a siliceous rock plainly depends on the state 



