This guide will help you recognize some geologic features of buildings, in addition to their historical and architectural aspects, wherever you travel. However, remember one important point when examining buildings and monuments for deterioration: stone deterioration has many causes. Although acid precipitation and urban pollution can accelerate stone deterioration, people, pigeons, and other organisms may also harm our stone structures. In addition, the process of weathering has been going on since the Earth first had an atmosphere. Although we can observe deterioration of the stone, it is hard to determine how much of the deterioration is from acid precipitation and how much is from other causes. [Illustration: Pigeons sitting on the statue heads have created distinctive deterioration on this building.] [Illustration: Flowers and grasses have grown in the cracks between stones on this church.] [Illustration: This limestone column in the Lincoln Memorial is darkened and dirty from people’s hands touching the stone.] [Illustration: Microorganisms have caused this stain to appear on a marble column at the Jefferson Memorial.] What are we doing about acid rain? Scientists from many disciplines are studying acid precipitation and its impact. The National Acid Precipitation Assessment Program (NAPAP), a Federal program involving representatives from more than a dozen Federal agencies, has sponsored studies on how acid rain forms and how it affects lakes, crops, forests, and materials. Because buildings and monuments cannot adapt to changes in the environment, as plants and animals can, historic structures may be particularly affected by acid precipitation. Scientists are studying effective control technologies to limit the emissions from power plants and automobiles that cause acid rain. The impact and usefulness of regulations that would require limits on air pollution are also being studied. Finally, scientists are examining the processes of deterioration to find effective ways to protect and repair our historic buildings and monuments. Agencies like the National Park Service, which are charged with protecting and preserving our national heritage, are particularly concerned not only about the impact of acid rain but also about making the best choices for maintaining and preserving our historic buildings and monuments. [Illustration: Beginning in 1984, the National Acid Precipitation Assessment Program sponsored exposure site studies of limestone and marble, to examine the contribution to stone deterioration that comes from acid precipitation.] A field guide to buildings in our Nation’s capital