He is protecting the town as a vigilante.

In order to protect the safety of the town, we organized a vigilante group.

This rock contains a lot of silica.

He was deeply thinking about the physical world.

He was punished for a minor offense.

This rock was formed through the process of silicification.

This fossil has been silicified over millions of years.

He floated a light boat on the lake.

He just made a small mistake.

He made a small mistake during the exam.