It isn't possible for fossils to form in the IGNEOUS ROCK. The heat along with the pressure of the magma would simply crush the bones. So to decide the period of sedimentary rock layers, scientists initially need to discover adjacent layers of Earth that incorporate IGNEOUS ROCK, for example, volcanic ash remains.
A push factor would be something like war or famine or something. A pull factor would be something like a job or new opportunity, its usually for financial reasons in my opinion. Hope this helped!
