What is the theory that america is a country of different cultures fusing together?

Melting Pot Theory. In cultural terms, America can be explained as a "melting pot", where people from different cultures immigrate to the United States and assimilate to the American culture.Yet, although it might be true that America is a multicultural country, it is less clear how well these immigrants assimilate to the American culture considering that most lead their lives within what's familiar (e.g. people tend to marry people of  their own race).

Where are the statements?