"He wore a small porkpie hat"
Buster Keaton wearing his trademark porkpie hat
Public DomainBuster Keaton wearing his trademark porkpie hat - Credit: The Blacksmith, 1922

A pork pie hat is made of felt or straw, and has a cylindrical crown and flat top. It originated in the mid-19th century, and was originally a ladies’ hat.  According to American fashion reports of the 1930's, the smooth dark brown felt was the original popular model, but the "fuzzier" green model came in close second. 

Today, pork pie hats are often associated with jazz and blues musicians and fans.