In 1669, Niels Stensen (1638-1686), better known then and now by his Latinized nameii Nicolaus Steno, formulated a few basic rules that helped him make sense of the rocks of Tuscany and the various objects contained within them. His short preliminary work, De Solido Intra Solidum Naturaliter Contento — Dissertationis Prodromus (Provisional report on solid bodies naturally embedded in other solids), included several propositions that have since become fundamental to geologists studying all kinds of rocks. 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.
The Principle of Superposition
In a sequence of strata, any stratum is younger than the sequence of strata on which it rests, and is older than the strata that rest upon it."...at the time when any given stratum was being formed, all the matter resting upon it was fluid, and, therefore, at the time when the lower stratum was being formed, none of the upper strata existed." Steno, 1669.
Today we restrict this principle to sedimentary rocks, which were understood differently in Steno's time. Basically, he deduced that rocks were laid down in vertical order just as sediments are laid down today, under water, with new on top of old. This principle allows us to piece together the succession of fossil life that defines much of the geologic time scale.
Principle of Initial Horizontality
Strata are deposited horizontally and then deformed to various attitudes later."Strata either perpendicular to the horizon or inclined to the horizon were at one time parallel to the horizon." Steno, 1669.
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. Today we know that some strata start out tilted, but nevertheless this principle enables us to easily detect unnatural degrees of tilt and infer that they have been disturbed since their formation. And we know of many more causes, from tectonics to intrusions, that can tilt and fold rocks.
Principle of Strata Continuity
Strata can be assumed to have continued laterally far from where they presently end."Material forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way." Steno, 1669
This principle allowed Steno to link identical rocks on opposite sides of a river valley and deduce the history of events (mostly erosion) that separated them. Today we apply this principle across the Grand Canyon—even across oceans to link continents that once were adjoined.
Principle of Cross Cutting Relationships
* Things that cross-cut layers probably postdate them."If a body or discontinuity cuts across a stratum, it must have formed after that stratum." Steno, 1669
This principle is essential in studying all kinds of rocks, not just sedimentary ones. With it we can untangle intricate sequences of geologic events such as faulting, folding,deformation, and emplacement of dikes and veins.