1938 to 1950 is called the Golden age of comics not because the first comics appeared as this art form had been around for a while.
It is refered to as the golden age as it was the dawn of the superhero.
During this time many notable firsts happened
Superman (action Comics 1)
Batman (Detective Comics 27)
Shazam (during this time bigger than Superman)
These are now the most expensive comic books to get with Action Comics 1 and Detective Comics 27 Selling for millions of dollars