[Illustration: All of the Federal buildings that form the Federal Triangle (between Pennsylvania Avenue, Constitution Avenue, and 15th Street) are made of limestone.] Continue south on 15th Street to Constitution Avenue. From the corner of 15th Street and Constitution, follow some of the foot paths half a kilometer (three tenths of a mile) to the Washington Monument. Washington Monument—Site 14 This monument was begun in 1848, but it was not finished until 1885; the change in color about 150 feet up marks a change in the type of marble used to face the monument. Although it is made of marble, its smooth, straight shape and the massive blocks used in this monument have minimized the effect of acid precipitation. Dissolution does occur in a few areas, but the amount of stone material lost from dissolution is insignificant compared to the mass of the stone. [Illustration: The straight shape and massive stones in the Washington Monument minimize the impact of acid precipitation to this important landmark.] Our tour ends here, but there are many more stone buildings and monuments in Washington and in other cities that may also show the effects of urban pollution and acid precipitation. However, as seen on this tour, not all of the deterioration of stone buildings is caused by acid rain. To protect our historic stone buildings and monuments, we need to limit air pollution that contributes to acid rain deterioration, and we need to develop effective maintenance and preservation procedures that will not further harm the stone. Glossary of Geologic and Architectural Terms Antefix—an ornament along the edge of a roof, often shaped like a shell or shield Baluster—a post or support for a handrail Balustrade—a rail and row of supporting posts, especially along a stair or porch Bronze—a metal alloy of copper and tin; although brown when fresh, it weathers to a more commonly seen green color Calcite—a mineral composed of calcium carbonate: CaCO₃ Column Capital—The top portion of a column. There are three main types; from least ornate to most ornate, they are Doric, Ionic, and Corinthian Diabase—a dark-gray to black, fine-textured igneous rock, composed of the minerals feldspar and pyroxene Feldspar—a common mineral composed of silica (Si), aluminum (Al), and calcium (Ca), potassium (K), or sodium (Na): CaAl₂Si₂O₈, KAlSi₃O₈, NaAlSi₃O₈ Fossil—remains or trace of a plant or animal preserved in a rock from some past, prehistoric time Granite—a light or speckled, coarse-grained igneous rock composed of quartz, feldspar, and mica Guttae—small, drop-like rounded ornaments under a roof overhang Gypsum—a mineral composed of hydrated calcium sulfate: CaSO₄·2H₂O Igneous—formed from melted or partially molten rock Inclusion—a mineral phase that is distinct (in composition or appearance) from the main mineral components of a rock