Three rules of relative dating
He also found that certain animals were in only certain layers and that they were in the same layers all across England.
Due to that discovery, Smith was able to recognize the order that the rocks were formed.
But in the case of minerals, he could assert that crystals accrete from outside rather than grow from inside.
This is a profound observation that has ongoing applications for igneous and metamorphic rocks, not just the sedimentary rocks of Tuscany.
" Steno was speaking very generally here, and he went on to discuss the growth of animals and plants as well as minerals, drawing on his deep knowledge of anatomy.
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.
In 1669, Niels Stensen (1638-1686), better known then and now by his Latinized name 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, (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.
The other principles are often called Steno's Laws, but this one stands alone at the foundation of crystallography.