Dark sand?
Pablo, I would try and remove any loose material, as you are right, it will hurt the adhesion if anything is loose. Brush down bothe surfaces so that they arte clean, before you glue them back together.
With contact cement, you need to put glue on both pieces you want glued together. Not too much, but just enough. Then you let it dry a little so that it becomes sticky, but doens't come off the piece - then you press the two pieces together. BUT make sure it is perfectly lined up, because it's almost impossible to pull aprat if you mess it up.
