. thus demonstrating the progress of reactions and modification of the fluid as it moved into the rock away from the vein or fluid pathway These observations demonstrate the dynamic and complex nature of hydrothermal systems, but they also provide important information which aids in interpreting the physical and chemical history of a hydrothermal system. During research on hydrothermal systems, the careful assessment of field relations, mineralogical and and analytical data are textural observations critical to forming a rigorous interpretation. The Atlas of Alteration will assist with some of these steps.