When a worn car-wheel tread is examined under the microscope it is perceived that the surface of the metal comes off in thin flakes or scales.

Examined under high powers the scales are found to resemble portions of a brick wall, the fractures not being in the particles of iron, but in the materials which unite the particles in a manner similar to which mortar unites the bricks of a wall. Continuous jarring breaks the cement or uniting material, thus allowing iron so treated to fall in pieces.