Could not the gospel writer have been a bit sarcastic in his use of the word, "rock?"  After all Peter was the least rock-like of the characters in his gospel.  Wasn't Jesus being a bit ironic in his statement?  Isn't the message that the person, who was to be the leader of the apostles, was a fallen, weak, very fallible man?  All the Gospels point that out.  Yet to our great comfort Jesus chooses him, a fallen man like us.  Certainly Peter should be humbled by the choice, but as the gospels go on he can still very mistaken, more fallible, can he not?  Is there perhaps a message there for all the members of the Church, that leadership is not granted only to the super holy?  We see often it is not.  "Rocks" like the one Jesus confers leadership on are all over the place in the Church, are they not?