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The United States Geological Survey (USGS) is a scientific organization created in 1879, and is part of the U.S. government. Their scientists explore our environment and ecosystems, to determine the natural dangers we are facing. The agency has over 10,000 employees that collect, monitor, and analyze data so that they have a better understanding of our problems. The USGS is dedicated to provide reliable, investigated information to enhance and protect our quality of life. This is one of their circulars. This work has been selected by scholars as being culturally important, and is part of the knowledge base of civilization as we know it. This work was reproduced from the original artifact, and remains as true to the original work as possible. Therefore, you will see the original copyright references, library stamps (as most of these works have been housed in our most important libraries around the world), and other notations in the work. This work is in the public domain in the United States of America, and possibly other nations. Within the United States, you may freely copy and distribute this work, as no entity (individual or corporate) has a copyright on the body of the work. As a reproduction of a historical artifact, this work may contain missing or blurred pages, poor pictures, errant marks, etc. Scholars believe, and we concur, that this work is important enough to be preserved, reproduced, and made generally available to the public. We appreciate your support of the preservation process, and thank you for being an important part of keeping this knowledge alive and relevant.