Which is heavier, milk or cream?

'Milk is heavier than cream. Cream in milk rises to the surface because it is composed of infinitesimal drops of oil and fat that are lighter than water and the rest of the components of milk. It is the same principle that makes oil float on the surface of water. The rising of cream is not at first apparent because the droplets are very small and they come to the surface slowly.'

From "A Book About A Thousand Things" by George Stimpson.