This small sandstone building was built around 1828 at the west entrance to the Capitol. In 1880 it was moved (along with a twin and four gateposts) to its present site. This building is made of the same sandstone that was used in the central part of the Capitol and in the White House. Three types of deterioration are readily visible at the gatehouse: spalling, pock marks, and preferential weathering of clay layers in the stone. This stone may be more degraded than stone in the Capitol or the White House, because of variations in stone quality and maintenance to the buildings and because it has never been painted. [Illustration: This kind of sandstone was soon found to be a poor building stone because of its tendency to spall. (detail on Capitol Gatehouse)] To continue, we will cross 17th Street and examine parts of several buildings as we walk north. Organization of American States Building—Site 8 [Illustration: The Organization of American States Building is made of marble and was dedicated in 1910.] This marble building was dedicated in 1910. Two sculptures in the front of the building show some alteration crusts in sheltered areas and dissolution in exposed areas. In back of the building the marble balusters on the patio are covered with blackened crusts, especially on the sides facing the garden. In many places the crusts have blistered or spalled off, exposing new surfaces to alteration. In general, the patio sides of the balusters are in much better condition than the sides that face the garden, perhaps because washing of the patio has washed off the gypsum crusts on that side of the balusters. [Illustration: Blackened gypsum crusts may blister and spall off, exposing a crumbling stone surface to further pollution.] Continue north through the garden and parking lot and cross C Street to the Daughters of the American Revolution (DAR) Buildings. DAR—Constitution Hall—Site 9 [Illustration: DAR—Constitution Hall is made of limestone and was built in the 1930’s.] The main damage on this limestone building, built in the 1930’s, is the blackening of the side balustrade from algae or fungi. The stone is porous and therefore retains moisture, thus encouraging growth of organisms. The limestone in this building is quite uniform and shows little preferential dissolution, except in a few places. On the top of the balustrade along C Street, for example, some of the calcite matrix has dissolved from around the fossil fragments, and some holes are filled with calcite crystals. [Illustration: Some of the blackening on limestone surfaces may be from algae or fungi that readily grow in the rough surface in Washington’s warm, humid climate.] Continue east along C Street to Memorial Continental Hall. DAR—Memorial Continental Hall—Site 10