I've got question for you first.
DUDE.. You're touring EUROPE with friends and/or on your own, and you're concerned about this one AMERICAN girl???
What about European Girls? Have you had one yet? Are you enjoying your trip or just spending the days and counting them till you see her again??
Dude, please, enjoy your trip, meet other women!!
Ok.
Now, about that FB thing and the 6 hours she didnt reply, she might be shit testing you.
She might do that on purpouse just to see how desperate or interested you are. Usually guys would text.."are you there?" "did you read my msg?" " Hello??"...But you won't
Another important thing is: Why is she traveling alone? Maybe she wants to feel like an independent lady adventuring and experiencing the world and the lone idea of being slightly attached to a dude she just met is disgusting to her (the idea of it).
Maybe she likes you but she wont give up that easy. I'd say freeze her out, enjoy your fucking trip, say hi on FB every now and then and when the day comes, just call or msg her and set up the date in scotland.
Seems to me like you miss her too much, and that's ok, I know that feeling too, but man, you're in Europe traveling. Do not over concentrate on this one girl man.