If you can recognise the layers as the history of a place,you can identify two place as having the same origins. In a very few instances, such as iridium brought in by a meteor or radioactive dust risen by an atomic bomb,material in the atmosphere can settle onto much of earth's surface in a relatively even blanket. In these very thin layers,there is a lateral continuity at a global scale. A fourth concept has been added to steno's three:the principle of faunal succession. This tells us that animals(and plants,and other fossilised biota) have changed over geological time and can be used to understand rocks. It could perhaps have more accurately been referred to as the principle of biological succession.