350 REPORT ON NATIONAL MUSEUM, 1886. 



cause ot this remarkable variation is little understood and can not here 

 be touched upon,* but the fact that such occurs is of importance, since 

 in many and perhaps the majority of cases an equal variation exists 

 in point of durability. By English as well as many other authorities 

 a dolomite is, other things being equal, considered more durable than 

 a limestone, and beyond doubt this is the case in localities where the 

 atmosphere is at all acidic, since dolomite, as already noted, is but lit- 

 tle affected by these agencies. Aside from this it would seem yet to be 

 proven that, in the United States, a pure limestone was less durable 

 than one that contained the necessary magnesia to constitute a true dolo- 

 mite, f Indeed, Professor Hall considers the magnesian limestones, as a 

 whole, " more friable, more porous, and less firm " (and consequently 

 less durable) than the pure limestone. t 



Stones which are mixtures of limestone and dolomite are liable to 

 weather unevenly, the limestone crystals becoming eaten out, while the 

 dolomite particles are left to project and impart a rough and lusterless 

 surface. 



Coarsely fossiliferous stones are usually to be avoided for exposed work, 

 as they weather unevenly, owing to the unequal hardness of the fossils 

 and the matrix in which they are embedded. Thus the coarse gray Niag- 

 ara limestone from Lockport, X. Y., used in the construction of the Lenox 

 Library building in New York City, began to show signs of decay even 

 before the structure was completed. It should be remarked, however, 

 that this extreme rate was due in part to the fact that the s#one was 

 laid on edge and not on the natural bed. Mr. Wolff § mentions a case 

 of a monument of shell marble in a Boston cemetery, in which, after sev- 

 enty years' exposure, the fossil shells stand out in bold relief; the stone 

 is also covered with fine cracks and is otherwise decomposed. || 



Veined stones are also subject to unequal weathering when exposed; 

 this being due to the unequal hardness of the vein matter and the mass 

 of the rock. This is true of all stones, but is especially noticeable in 



* Interested parties should eonsnlt such works as Geikie's text book of Geology and 

 Prestwich's Chemical and Physical Geology and the authorities there alluded to. 



t "The nearer a magnesian limestone approaches a dolomite in composition the 

 more durable it is likely to be." "hx the formation of dolomite some peculiar com- 

 bination takes i^lace between the molecules of each substance; they possess some 

 inherent power by which the invisible or minutest particles intermix and unite with 

 one -another so intimately as to be inseparable by mechanical means. On examining 

 with a high magnifying power a specimen of genuine magnesian limestone * * * 

 it will be found not composed of two sorts of crystals, some formed of carbonate ot 

 lime and others of carbonate of maguesia, but the entire mass of stone is made up 

 of rhomboids, each of which contains both earths homogeneously crystallized to- 

 gether. When this is the case we know by practical observation that the stone is 

 extremely durable." (Smith's Lithology, Building Const., p. 40.) 



I Rep. Tenth Census, p. 290. 



§ Rep. on Building-Stone, p. 40. 



UThe limestone of which was constructed the State capitol building at Nashville, 

 Tenu., has proved so inferior, owing to the weathering out of tho numerous fossil 

 orthocera, that the quarries have been discontinued on this account alone. 



