350 REPORT ON NATIONAL MUSEUM, 1886. 



cause of tliis roinarkable variation is little luulerstood and can not here 

 be touehed 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 lin)estone was less durable 

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

 mite. ^ Indeed, Professor Hall considers the magnesian limestones, as a 

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

 less durable) than the i>ure limestone.| 



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, N. 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 coinpleted. It should be remarked, however, 

 that this extreme rate ^Tas due in ])art to the fact that the s(»one was 

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

 of a monument ot 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 w^eatheriug 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 



* lutorested parties should cousult such works a,s Geikie's text book of Geology aud 

 Prt'i^twich's Chemical and Physical Geology and the authorities there alluded to. 



t "The nearer a maguesiau liiuestouo approaches a dolomite in comiiositiou the 

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

 bination takes place between the molecules of each substance; they possess some 

 inherent jiower 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 oi 

 lime aud others of carbonate of magnesia, 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. 



S^ Rep. on Building-Stone, p. 40. 



II The limestone of wliioji was constructed the State capitol building at Nashville, 

 Tenn., has i^roved so inferior, owing to the weathering out of the numerous fossil 

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



