Three rules of relative dating
He put forth three propositions, the first being this: "If a solid body is enclosed on all sides by another solid body, of the two bodies that one first became hard which, in the mutual contact, expresses on its own surface the properties of the other surface." (This may be clearer if we change "expresses" to "impresses" and switch "own" with "other.") While the "official" Principles pertain to layers of rock and their shapes and orientations, Steno's own principles were strictly about "solids within solids." Which of two things came first? Thus he could confidently state that fossil shells existed before the rock that enclosed them.
And we, for example, can see that the stones in a conglomerate are older than the matrix that encloses them.
Three of these are known as Steno's principles, and a fourth observation, on crystals, is known as Steno's Law.
The quotes given here are from the English translation of 1916.
"If a solid substance is in every other way like another solid substance, not only as regards the conditions of surface, but also as regards the inner arrangement of parts and particles, it will also be like it as regards the manner and place of production...
Due to that discovery, Smith was able to recognize the order that the rocks were formed.
This principle allows us to piece together the succession of fossil life that defines much of the geologic time scale.
Steno reasoned that strongly tilted rocks did not start that way, but were affected by later events—either upheaval by volcanic disturbances or collapse from beneath by cave-ins.
Though relative dating can only determine the sequential order in which a series of events occurred, not when they occurred, it remains a useful technique.
Relative dating by biostratigraphy is the preferred method in paleontology and is, in some respects, more accurate.