Selenite (mineral) was filled with mineral-rich water that drove the crystals' growth. The largest of those crystals weighs 55 tons, is long, and is over 500,000 years old. Gypsum occurs on every continent and is the most common of all the sulfate minerals. Gypsum is formed as an evaporative mineral, frequently found in alkaline lake muds, clay beds, evaporated seas, salt flats, salt springs, and caves. It is frequently found in conjunction with other minerals such as, copper ores, sulfur and sulfides, silver, iron ores, coal, calcite, dolomite, limestone, and opal. Gypsum has been dated to almost every geologic age since the
Vegetation and lawns; all are encompassed by the term "vegetation". The vegetation type is defined by characteristic dominant species, or a common aspect of the assemblage, such as an elevation range or environmental commonality. The contemporary use of "vegetation" approximates that of ecologist Frederic Clements' term earth cover, an expression still used by the Bureau of Land Management. Natural vegetation refers to plant life undisturbed by humans in its growth and which is controlled by the climatic conditions of that region. The distinction between vegetation (the general appearance of a community) and flora (the taxonomic composition of a community) was first