* 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.
* 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.
* 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
* 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
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