BUILDING STONES. 187 



General conclusions.] 



made with them. In these observations, as in those of the preceding tests not depending on the 

 balance, correctness depends much, of course, on the judgment of the operator. 



(C.) GENERAL CONCLUSIONS RESPECTING THE QUALITIES OF 

 BUILDING STONES. 



The weathering of a building-stone which results in its final demolition 

 is partly physical and partly chemical. These processes aid each other. A 

 stone that is easily rent by frost, or is washed and eroded rapidly by rains or 

 hail, or wind, will quickly prove an element of weakness in a building; 

 and one that is able to resist these physical agents successfully, when aided 

 by the invisible action of rapid chemical change, will also soon disintegrate 

 when favorably exposed to all these forces. For instance, if a limestone be 

 so placed in a building as to be protected from the physical action of wind 

 and rain, it becomes coated with a film of dirt which results both from its 

 own chemical change and the accumulation of dust. This film acts to pro- 

 tect the stone from the chemical changes due to the vapors or acids that 

 float in the air, or that are in solution in rainwater; if, however, the same 

 stone be freshly washed by every pelting rain, its corners become rounded 

 and its entire exterior surface slowly wears away, under the unobstructed 

 action of both chemical and physical forces. The same is true of sandstones, 

 especially those having a calcareous cement, and also of the crystalline 

 rocks. In the case of granite, however, the change is so slow that the co- 

 operation of chemical and physical forces can only be seen and estimated 

 in their natural beds where there has been time sufficient for the change of 

 one mineral to another by the substitution of different elements and the 

 removal of some of those which were there at first. After this change has 

 been effected, if physical causes remove the weakened mineral a fresh sur- 

 face is presented for the continuance of a slow chemical change. The 

 glacial epochs have thus operated to keep the crystalline rocks fresh in 

 northern latitudes, while further south, and beyond the limit of glaciation, 

 the decayed material of the crystalline rocks has frequently accumulated 

 to great thicknesses.* The crystalline rocks of Minnesota, so far as they 

 have been quarried, and tested by the survey, seem to show the freshness 

 of the glaciation that has passed over them in their remarkable strength 

 under pressure. 



"Compare the report of K. W. Raymond on tile mining resources of the United States, 1874, p. 335. The Silver City 

 mining district of New Mexico. 



