f 210 [Assembly 



enduring limestones we liave are magnesian in character, having such 

 proportions of lime and magnesia as constitute a dolomite. This is 

 the character of the Niagara limestone and of some of the older lime- 

 stones of the Silurian series, both in their normal and meLamorphic 

 condition. As a general rule, however, the magnesian limestones, in 

 their normal condition, are more friable, more porous and less firm in 

 their character than the pure carbonates of lime. The presence of iron 

 in magnesian limestones, either as an oxide or a carbonate of iron, may 

 often aid in hastening their decomposition. They usually weather to 

 a brownish hue, which is sometimes yellowish or drab-colored, but 

 more often, in the unaltered condition, to an ashen gray. The yellow- 

 ish color is due to iron in some form, either as an oxide or a carbonate. 



In the selection of limestones for structures of any kind above ground, 

 care should be taken to avoid the shaly seams which are the principal 

 cause of decay ; and though the stone containing them may endure for 

 many years, they yet present an unsightly appearance. We have, in 

 the city of Albany, a good example of this in the wails of the Reser- 

 voir on Eagle street in Albany ; and numerous other cases of similar 

 character might be cited. In all these examples, it may be observed 

 that the dilapidation comes from the cause specified, and no other ; 

 for in most of the structures exhibiting this defect, the tool-marks are 

 not yet obliterated from the surface of the solid limestone. 



Limestones of this character, however, are perfectly safe and fit for 

 any foundation or other work placed beyond the reach of freezing and 

 thawing ; and they possess a strength and power of resistance to pres- 

 sure, which fits them for the heaviest structures. 



Although limestones, in their normal condition, as well as the 

 marbles, are liable to decay from the action' of rain-water charged 

 with carbonic acid, yet this cause usually operates so slowly on the 

 walls of a building that the tool-marks are rarely obliterated in a quar- 

 ter of a century.* The more porous limestones, and some of the 

 marbles which notoriously lack cohesive power, may be more aSected 

 by this action. The liability to be decomposed and disintegrated by 

 this process is always sufficiently shown in the natural surfaces of 

 quarries ; and in some cases we find the exposed beds crumbled to a 

 mass of sand, while the layers beneath the reach of water and frost 

 are comparatively solid, f 



Granite and Granitic Rocks. 



In the extensive class of rocks coming under the head of Granites, 

 the conditions of durability and causes of decay are .somewhat modi- 

 fied by the chemical changes which have supervened among the original 

 mechanical aggregations, and the crystalline character which they 

 have assumed. In these rocks we have quartz, felspar, mica and 



♦ The dark compact limestone at the base of the Lower Helderberg group, in some speci- 

 mens in exposed situations, has retained the tool marks for nearly a century ; and lettering 

 cut on blocks of this stone, more than a century since, are Rtill fresh and well defined. 

 These examples may be seen in an old church in Schoharie, known as the Old Fort, 

 from having been thus occupied during the revolutionary war ; and in the Lutheran Church 

 near the Court-house, where some lettered stones, from the first church erected in that 

 town, have been laid in its foundations. 



+ In this process, the water dissolves a email portion of the stone as far as it reaches, 

 and thus separates the particles still more; and the further access of water, which freezes 

 in the stone, produces a rapid disintegration of the mass. 



