BUILDING AND ORNAMENTAL STONES. 34 ( J 



The porosity of any stone is usually characteristically shown by its 

 manner of drying after a rain ; some will dry quickly, while others that 

 have absorbed a larger quantity of water will remain moist for a long 

 time. In the ease of a sandstone it may be said that the grains should 

 be closely compacted, so that the proportion of cement necessary to en- 

 tirely till the interspaces is comparatively small. Of all cementing ma- 

 terials the argillaceous and calcareous are the least durable, and the 

 purely siliceous the most so, the ferruginous cements standing interme- 

 diate in the series. Indeed a purely siliceous sandstone cemented 

 closely by a siliceous cement may be classed as one of the most durable 

 of stones, although unfortunately on account of their hardness and poor 

 colors such can be utilized only at a considerable expense and not al- 

 ways with good effect. Professor Geikie * mentions an instance in wh icl i 

 a line siliceous sandstone erected as a tombstone in Grey friars church- 

 yard about 1G4G, and defaced by order of the Government in 10(52, still 

 showed the marks of the defacing chisel upon its polished surface after 

 a lapse of over two hundred years. 



(3) COMPARATIVE DURABILITY OF STONES OF VARIOUS KINDS. 



hi this connection the following table upon the "life" of various 

 kinds of building stone in New York City is of interest; by the term 

 life being understood the number of years that the stones have been 

 found to last without discoloration or disintegration to the extent of 

 necessitating repairs. 



Liio iu years. 



Coarse brown-stone 5 to 15 



Fine laminated brown-stone 20 50 



Compaet brown-stone , 100 200 



Biue-otone (sandstone), untried, probably centuries. 



Nova Scotia sandstone, untried, perhaps - 50 200 



Ohiosandstone (best siliceous variety), perhaps from one to many centuries. 



Coarse fossiliferons limestone 20 40 



Fine oolitic (French) limestone 30 40 



Marble, coarse dolomitic 40 



Marble, line dolomitic '-- 00 80 



Marble, fine 50 100 



Granite 75 200 



Gneiss, 50 years to many centuries. t 



The fact that certain quarries have furnished good material in the 

 past is no guarantee of the future output of the entire quarry. This 

 is especially true regarding rocks of sedimentary origin, as the sand 

 and limestones, different beds of which will often vary widely in 

 color, texture, composition, and durability, though lying closely adja- 

 cent. In many quarries of calcareous rocks in Ohio, Iowa, aiu\ neigh- 

 boring States, the product is found to vary at different depths all the 

 way from a pure limestone to magnesian limestone and dolomite. The 



** Geological Sketches, p. 175. 



t Julien, Rep. Tenth Census, 1880, p. 391. 



