No. 104.] 209 



if not entirely weathering out, some parts of the lines or bands of color 

 are more susceptible to the action of the weather, because unevenly 

 disintegrated, and finally present an unsightly surface. Bands or 

 stripes of color, in all the marbles, indicate a different texture and com- 

 position from the other parts of the mass, and all examples of this 

 character will weather unequally. Such stones, therefore, should be 

 used with great caution in all structures intended to be permanent. 



In some of the marbles there are numerous spots of soft talc-like sub- 

 stance, which weathers more easily than the surrounding stone. These 

 will either weather to different color, or from softening readily on 

 exposure, give opportunity for the growth of minute lichens, thus 

 covering the stone with dark specks or blotches. Under other cir- 

 cumstances these spots may be of different color, but scarcely less un- 

 sightly, and in the end working the gradual dilapidation of the stone. 

 The white marble of Lee in Massachusetts is everywhere marked by 

 these talcy spots, and the monuments and gravestones in the ceme- 

 teries of the neighborhood are covered wijth black specks and blotches. 



The marbles, however crystalline they may be, are not free from the 

 same impurities that affect the unaltered limestones ; and iron pyrites 

 occurs in these, both as segregated veins or lines of accumulation, 

 interrupted strings or nodules, and disseminated in minute particles 

 throughout the mass. A good example of the latter may be seen in 

 some marble at Sheffield in Massachusetts, where the stone contains 

 minute particles of iron pyrites, which, becoming decomposed on 

 exposure, gives to the entire surface a slight rusty hue. The same 

 change supervenes in the dressed marble ; and some of the blocks in 

 the New City Hall of New York show the rusty hue immediately after 

 having been laid in the wall. This may be a case in which the change 

 will cease after a time, for want of access of moisture to the interior 

 portions, or by the filling of the jDores with sulphate of lime produced 

 by the decomposition of the pyrites, and thus protecting the deeper 

 portions of the stone. 



Besides the ordinary seams or lines of color in the direction of the 

 bedding, many of the marbles are marked by the presence of irregular 

 veins or lines of segregation, which are different in composition and 

 texture from the surrounding rock, and though sometimes not very 

 different in color, and, therefore, showing little in the outset, will 

 nevertheless usually decompose more readily than the adjacent stone. 

 Veins of this kind are of common occurrence in some of the marbles 

 used for building, and may be observed in their full effect in the State 

 Hall and City Hall of Albany. These veins usually consist of some 

 soft talc-like mineral with magnesian limestone and iron. The pure 

 white marble, free from seams or veins of any kind, constitutes the 

 smallest part of any or all marble quarries. The columns in front of 

 the " old United States Bank," in Philadelphia, offer one of the best 

 exami)les of the destruction of marble from the several causes men- 

 tioned. Although erected scarcely fifty years since, the bedding seams 

 are Aveathered and opened to such a degree as to present an aspect of 

 extreme dilapidation, and less than half a century more will effect 

 their entire destruction. 



The simple presence of magnesia alone does not necessarily impair 

 the enduring quality of a limestone. Some of the hardest and most 



[Assem. Doc. No. 104.) 27 



