Marl, a natural mixture of clay and calcium carbonate. It is a loose, earthy substance, usually gray or bluish in color. Marl deposits are formed in shallow lakes. The calcium carbonate marl is derived from the biological action of certain aquatic plants on dissolved carbonates. Shell marl contains shellfish remains. Greensand marl contains glauconite, or greensand, a silicate of potassium and iron. Marl is used to neutralize acid soils. Portland cement is sometimes made with marl instead of limestone.
Comets are remarkable pieces of our universe's past, and they tell us a great deal about how the universe was formed. Learn about the long but rewarding process of discovering and analyzing comets.
The Smithsonian Institution began with an unexpected donation and has now grown to a massive collection of American artifacts. The Smithsonian is world renowned for its museum and for scientific endeavors. Take this quiz and learn more about the famous Smithsonian Institution.