216 [Assembly 



mixture of fine grains of sand with pebbles of various sizes, does not 

 usually endure well. Similar sandstones or conglomerates, when par- 

 tially metamorphosed, and cemented by silica, or some siliceous com- 

 pound, are less affected by the weather and are more durable. In the 

 crystalline marbles, some of the coarser varieties are weak from the 

 want of cohesion or cementing matter between the crystals. The same 

 is equally true occasionally of those which are more finely crystalline ; 

 and we sometimes find a coarsely crystalline marble stronger than a 

 finer one, in similar beds but a few miles asunder, or even beds in the 

 same quarry may diflFer in this respect. The coarsely crystalline marble 

 of Tuckahoe is stronger than the finer-grained marble of Sing Sing 

 and other places in the neighborhood. So far as the marbles are con- 

 cerned, all the crystalline forms, be they coarse or fine, may be strong 

 or weak. The fine-grained marbles, which show scarcely a crystalline 

 structure, or such only as the calcareous muds might take on in their 

 nietaniorphism, are the most durable stones of this kind. 



6. Cementing materials. I have already alluded to this feature 

 under the preceding head. When the cementing material is clay, or 

 where argillaceous matter predominates, it is ra2:)idly disintegrated by 

 the absorption of water, and freezing and thawing while thus saturated. 

 Where the cementing matter is calcareous, it will dissolve more slowly, 

 and only through the agency of rain water carrying carbonic acid. 

 Where the cement is siliceous, it is essentially indestructible from the 

 effects of the atmosphere and water. 



The cementing material of the Tertiary sandstones of which the 

 Old Capitol, Treasury and other buildings in Washington were con- 

 structed, is clay and carbonate of lime, and its rapid disintegration 

 from rain and frosts is always observable. As before noticed, some 

 friable sandstones become harder on exposure, and this change has 

 been presumed to be due to the formation of a siliceous cement on 

 and near the surface. Sometimes probably a silicate of lime, or a 

 small quantity of calcareous matter held in solution in the interstices 

 of the stone, may become precipitated as solid carbonate of lime, in 

 accordance to a well-know law, on exposure to the atmosphere. 



Every geologist knows that not only sandstones, but all other rocks 

 are more easily shaped and trimmed when freshly broken from the 

 ledge or quarr}', than after they have remained for some time exposed 

 to the atmosphere or even carefully packed. The hardening or tough- 

 ening process, however, extends but a little way beneath the surface, 

 and the interior of a block remains essentially as when first quarried. 



IX. 



Causes affecting the Durability of a Stone, "which are ac- 

 cidental OR DUE TO Artificial or Extraneous Conditions. 



Many stones, which with proper treatment or under favorable cir- 

 cumstances might prove a durable building-material, are brought to a 

 rapid decay by conditions to which they are subjected in the structure. 



1. Tlie action of freezing and thaiviny. This alternating process of 

 freezing and thawing is the most trying to the durability of a stone, of 

 any or all the conditions to which it is subjected. Of course this de- 

 pends upon the climate or latitude in which the stone is exposed. The 



