Mud is a mixture of water and various types of soil, silt, and clay. Over geological time, ancient mud deposits solidify to produce sedimentary rocks such as shale or mudstone (generally called lutites). Sand is a granular substance that occurs naturally and is made up of finely split rock and mineral particles. Shells and bones are also considered sand. Coarse sand is used for making concrete, while fine sand is used for glass painting and other crafts. The word "mud" comes from the Old English mōd, which itself comes through the Latin mucca, meaning "she-goat". The term "sand castle" is first recorded in 1611 as "a little island of sand built by the sea" and refers to an artificial mound of dry sand used for coastal defense. Muds differ from sands in their color and composition. Sands usually have a white color due to the small size of the grains. In contrast, muddy waters tend to be brown or black due to the presence of organic material from decaying plants and animals. Sands are usually composed of silicon dioxide (45% by weight), with smaller amounts of iron, magnesium, aluminum, and potassium. Mud consists of water mixed with organic matter and minerals such as phosphorus, sulfur, and calcium. In terms of volume, sands can be many times more abundant than mud because they pile up on shoreline formations or get washed away by floods to form new beaches.