The hard sciences have already been well represented in the thread. One could argue that any PhD in physics is a genius in terms of raw intellectual power. But I don't think listing Mensa or physics faculty was the purpose of OP; after all, Teddy Roosevelt was included in the list.Your view of genius is far too narrow for purposes of this thread.
We'll have to break this down a bit. Pure IQ points is not going to do it, because there are plenty of geniuses who you'll never hear about. So it has to include accomplishment, too. That being said, there are 10 American geniuses.