Ko. 104] 315 



while the former resists the penetration of fluids. In some of the 

 crystalHne limestones, the cohesion is so slight that the water admit- 

 ted, and freezing, has gradually broken np the mass, and we have a 

 bed of calcareous sand, of several feet in thickness, lying above the 

 rock which yet retains its ordinary consistence. Some of the fine- 

 grained and compact mechanical aggregations of rocks resist the 

 absorption of water in a remarkable degree. 



3. Ai-gillaceoiis matter in distributio)i or in seams. I have already 

 shown that the presence of a considerable proportion of argillaceous 

 matter distributed throughout the mass, be it calcareous or siliceous, 

 has a tendency to weaken and destroy the stone. Its presence in 

 seams or thin laminae produces the same result, as we have numerous 

 examples to show.' 



4. Iron 2^yrites {sulphuret of iron) and other foreign substances. 

 Iron pyrites (sulphuret of iron), whether intimately permeating the 

 stone or occurring in masses, layers or irregular nodules, is more or less 

 injurious and destructive. When not immediately destructive, 

 its decomposition renders the surfaces unsightly by staining the stone, 

 and finally breaking or disintegrating it "wherever this mineral occurs. 

 When disseminated through the mass, as it frequently is, it produces 

 slow but entire disintegration. 



It is not an uncommon thing to find masses of rock, in their native 

 position, completely disintegrated or softened to the depth of several 

 feet by decomposing iron pyrites. This feature is especially observable 

 in the gold region of Virginia, North Carolina and other Southern 

 States. In numerous instances, and sometimes over wide areas of 

 country, the rocks containing iron pyrites are decomposed by percolat- 

 ing rain water, to the general water-level of the surrounding country. 



In limestones or dolomites, the presence of iron pyrites operates disas- 

 trously ; for if magnesia be present, the sulphuric acid from the decom- 

 posing iron pyrites produces a soluble efflorescent salt, which exudes 

 to the surface and forms white patches, which are alternately washed 

 off and replaced, but leaving a whitened surface probably from the 

 presence of sulphate of lime. If the limestone be entirely calcareous, 

 the salt formed (a sulphate of lime), is insoluble, and therefore produces 

 less obvious results. 



In some cases, however, the lime of which the mortar or cement is 

 made may contain magnesia, and the decomposition of the iron pyrites 

 in the adjacent stone produces an efflorescent salt which exudes from 

 the joints. This condition is not unfrequently observed in buildings 

 constructed of the blue stone of the Hudson river group. As an ex- 

 ample, we may notice the efflorescent patches jDroceeding from some of 

 the joints between the stones of St. Peter's Church on State street in 

 Albany. 



The presence of iron in a low degree of oxidation tends to the de- 

 struction of the stone containing it. This is observed in the greenish 

 shales and sandstones and in some other rocks ; and this condition of 

 iron, as well as in the form of a sulphuret, may do much injury where 

 it exists. 



5. Size of constituent grains or particles. This feature has already 

 been alluded to under the head of granites, sandstones, etc. When 

 the separate minerals of a granite are in large crystalline masses, it is 

 an objectionable feature and a cause of decay. Coarse sandstone, or a 



