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single inch from a true level; in short, a building material of unsurpassed 

 durability and uniformity, and to which, as to the finer-grained marble in 

 the Clarksville quarries, no possible objection, except on the score of 

 expense, could be found, unless, indeed, it be considered one, that in this 

 material the effect of light and shade from projecting surfaces is in a 

 measure lost, while in marble and good tinted freestone every shadow is 

 sharply marked. 



3d. That the Aquia creek freestone, heretofore used in public buildings 

 in Washington, is a material not to be trusted to, being pervaded by dark 

 specks of the protoxide and peroxide of iron, which in peroxidating acquire 

 ayelloY/ish or reddish color, and having occasional clay holes, such as dis- 

 figure the Treasury and the Patent Office. A portion of this freestone was, 

 indeed, considered durable and free from material blemish; but the chance 

 of actually procuring it free from disfiguring spots and stains was consid- 

 ered so uncertain, that it was recommended to refrain from using it in the 

 institution building. 



4tli. That the freestone of the upper Potomac, in the vicinity of Seneca 

 creek, and found in quarries close to the line of the Chesapeake and Ohio 

 canal, is the best and most durable of all the Potomac freestones. 



The lilac-gray variety found in the Bull Run quarry, twenty-three miles 

 from Washington, was especially recommended, and pronounced to be 

 equal, if not superior, to that supplied for Trinity church, New York, from 

 the quarries of New Jersey. 



In regard to this latter material, it was stated that it possessed a quality 

 that should especially recommend it to the attention of builders. When 

 first quarried it is comparatively soft, working freely before the chisel and 

 hammer; but by exposure it gradually indurates, and ultimately acquires 

 a toughness and consistency that not only enables it to resist atmospheric 

 vicissitudes, but even the most severe mechanical wear and tear. Thus, 

 on the tow-path of the aqueduct, near Seneca creek, over which horses and 

 mules have been travelling almost daily for upwards of twenty years, this 

 freestone was found still unimpaired. Even the corners around which the 

 heavy lock-gates swing, showed no signs of chipping or decay; and on the 

 perpendicular wall of the acjueduct, where the water is continually oozing 

 through the joints and trickling down its face, forming an incrustation of 

 carbonate of lime, this freestone was observed, where the calcareous crust 

 had scaled off, with the grooves and ridges of the surface still nearly as 

 distinct as when the blocks first came from the hands of the stone-mason, 

 more than twenty years ago. 



The rare and valuable quality possessed by this freestone, of hardening 

 by exposure to the weather, and which may be due to iron in its compo- 

 sition, passing from a lower to a higher degree of oxidation, is occasionally 

 found in building-stone on the continent of Europe; as, for example, in a 

 calcareous freestone which has been excavated for centuries from St. Peter's 

 mountain, near Maestrich, in Belgium. It is highly prized wherever found, 

 as this peculiarity permits the freestone to be wrought at considerably less 

 expense than either granite or marble, and imparts to it a durability increas- 

 ing with age. 



Further to test the durability of these various building materials under 

 exposure to the vicissitudes of the seasons, specimens of each, and also of 

 other building-stones from New York and elsewhere, were handed to a 

 gentleman of this city experienced in chemistry, and having a laboratory 



