- In the basement of the apartment building there is a wall where people put their feet against the wall and leave marks.
- Siliceous rock (Wikipedia)
Siliceous rocks are sedimentary rocks that have silica (SiO2) as the principal constituent. The most common siliceous rock is chert; other types include diatomite. They commonly form from silica-secreting organisms such as radiolarians, diatoms, or some types of sponges.
