BUILUIN(; AND OiiNAMENTAL STONES. 349 



The porosity of any ytouc is usually cliaiacteristically shown by its 

 manner ot dry in <^- atter a rain ; some will dry quickly, while otiiers that 

 have absorbed a larger quantity ot water will remain moist lor a long 

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

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

 tirely lill 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 which 

 a fine siliceous sandstone erected as a tombstone in Greyfriars church- 

 yard about IGIG, and defaced by order of the Government in 1G(»2, still 

 showed the marks of the defacing chisel upon its polished surface af!' r 

 a lapse of over two hundred years. 



(:?) COMPARATIVl-: DIJKAIULITY OF STONES OF VARIOUS KINDS. 



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

 kinds of building stone in :s'ew York City is of interest; by the term 

 life being understood the number of years that the stoiu'S have been 

 found to last without discoloration or disintegration to the extent of 

 necessitating repairs. 



Life in years. 



Coarse Ijrowu-stoiie ^ to 15 



Fiuo l;uuin;ited browii-stouc 5iO JJO 



Coiiipiict brown-stone - 100 200 



Bluo-.stone (sandstone), untried, prol)a!)ly centuries. 



Nova Scotia sandstoce, iintricd, perliaps •'^■0 ~00 



Obiosandstone (best siliceous variety), perhaps Iroin one to many centuries. 



Coarse fossiliferous limestone ~0 40 



Fine oolitic (Frencb) limestone ^0 40 



Marble, coarse doloinitic 'JO 



Marble, line dolomitic tJU 80 



M arble, (1 ne , T.O 100 



Crauite 75 200 



Gneiss, 50 years to many centnries.t 



The fact that certain (piarries 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 wi<lely in 

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

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

 boring States, the product is found to vary at ditferent dei)ths all tbe 

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



* Geolo,u;ical Sketches, p. 175. 



t Julieu, Rep. Tenth Ceusns, IbSO, p. 391. 



